The sets and reps are hard to describe without having worked with the athlete for a bit, so please forgive me if what I say isn't a perfect fit but I will have a go.

First teach your body the movements without great stress as it will pay dividends later.

At first 2 or 3 sets is a good idea and initially starting with a lighter weight and a higher number of reps is good. You want to learn good form and it is hard to learn to do anything well if you are pushed close to max, so be humble in what you pick up in your first efforts.10, 12 or even 15 reps would be OK at first in 2 or 3 sets. Going to a gym will let you ask people about your form but do also watch U tube stuff because you will get detailed 15 minute explanations of how and why not just some gym jock's 30 second take on it.

View the initial period as learning the exercises, not as strength training, though it will make you stronger and you will get some slight hypertrophy too. To incentivise yourself by setting goals you can progress through, you could start with a week of finding out what weight you can just do 15 reps with. Next week do 3 sets if you can, 2 if you can't of 15 reps with the best style you can. When you think the style is right and the guys around you do too, then add the smallest plate you can find to each end of the bar, see what this does to the number of reps in the third week. Maybe you can do 12, maybe 10? Consolidate there for a week or two. Then add a plate. Does this land you around 3 sets of 8? Hopefully this will have been a fun progression where you see yourself moving through stages but also learning important stuff.

Remember that you are going to get stronger muscles but also your muscle recruitment will get better and this will make you quite a bit stronger too. If you start with the lighter weights, higher reps and full range of motion, the learned muscle recruitment has a better chance of learning the right patterns to then get all fibres to fire together on which will save you trouble later with fibres all firing together on poor patterns.

5 weeks is long enough for an athlete to deserve an easier week, 4 weeks is often the period chosen for a harder training block because it fits to a month but 5 is about the biological amount and by the time you take a week or 10 days of gentler stuff (A day or two off? a set less? 2 reps less?), you are messing up the round months anyway. When you have done at least 5 weeks of getting used to things and had at least a lighter week to consolidate (could be 2 cycles if that felt right -8 to 10 weeks of training with 2 weeks easier), then if you feel ready try the strength side a bit.

For strength training we need hard reps in small quantity with good quality. Experienced athletes may strength train with 5 or 4 reps in a set. I would say start with 6. Since the reps are few, adding the extra set after at some stage to make 4 sets is a good idea if you can do it. Try putting a set of a totally different exercise between sets to allow a fuller recovery.

So work to a position where you are doing sets of 6 hard, quality reps, ideally 4 sets, 3 is OK if not, particularly in a younger man (your young muscles take hints better than my older ones, sadly). The last set should be pretty hard but not with your form collapsing. As you get better you will find the last set is easier. Then its time to add a rep somewhere in the sets, then another and so on till you are doing 3 or 4 full sets of 8 reps and almost failing. Great, you made it. Clear demonstrable strength gains. This training thing is working. Now stay there and consolidate till comfy, even drop back a week to 7 or 6 reps, then add the next plate. This will probably land you back on fighting for the 6th rep. If you can only do 5, no worries. Just work away as before till you do all your sets up to 8 reps, then consolidate, add the next plate and pat yourself on the back for a good job well done.

You could eventually move to 4 sets of 4 that grow to 6, then you add a plate but I think its wiser to start with sets of 6 growing to 8. By the time you master that you will be clearly progressing and have learned enough to understand what works for your body.

Nutrition -yeah not too processed, plenty of variety, quality protein not protein shakes if you can help it. Fresh eggs, fresh fish, fresh meat, wild meat if you hunt, lots of greens, quality carbs not terribly processed. Its not rocket science but it can be a bit of work to change habits. Supplementation -most of its bollocks, some of it dangerous bollocks, some of it expensive bollocks. If you fancy something and it makes you happy -like say cordiceps mushroom- it probably makes you perform a bit better. If it has no serious side effects why not? but there is little that is proven to really work with no negative effects either. A scoop of creatine a day is probably a good thing. Its cheap, works and it appears to not have any ill effects after decades of testing. Stay the hell away from the small dick stuff like the anabolics. Your health is worth more.

Another thing is that in what I wrote earlier I said "lactate threshold" when I should have said "aerobic threshold" I mean the point at which lactate starts to gradually mount in the blood after an initial dip not the point at which its starts to shoot up to spike. An athlete with good mitochondria should show an initial dip in lactate as exercise load rises as lactate is the favourite fuel of muscles and brain and the mitochondria are just switching from things like fat to their favoured food as it becomes more available as a waste product from the fast twitch muscles. Cross fit guys/ gym class sort of guys are often very poor at this as they mostly condition upset fast twitch muscles and don't do the long slow work to build the aerobic system. I would condition strength and the aerobic system adding a sensible amount of high aerobic work (like your boxing for example) in the middle to make the heart hammer but not so much as to regularly load the muscles with more lactate than they can devour that burns them with hydrogen ions.

I hope that helps. Like I say it will be a bit specific to you in the end, so don't worry if you do different. Ask away if you like. But overall, first learn the good form and full range of motion of big compound exercises (deadlifts will do you so much more good than say biceps curls), then move to small sets of hard reps. A strong aesthetic body does not require isolating muscles and working the crap out of them (not much in life does this). Isolating muscles really helps hypertrophy but you want them to all work together in balance and harmony, to have useful strength, so big compound exercises are the way to get the biggest gains.

Enjoy.

First, eating/diet is number 1, by far. It doesn't have to be low calorie. Everything depends on your protein, macro- and icro-nutrients intake, which works with your metabolism.

Weight gym is best for bulk by a huge amount. Hence the hit the gym comments by everyone. Boxing and Mixed Martial Arts have huge injury problems. But, on the flip side are highly Alpha Male, self-assured, confidence like nothing else that you can do besides actual combat training.
Same injury issues with repetitive impact exercises or sports. Jump rope, running, soccer, basketball, football, etc. But can be beneficial for fun and althletics based on interest. Swimming is lowest impact with the best cardio. Next is cycling, but neither will build bulk or muscles, just muscle density and endurance.

Hope that helps.

I don't rate Crossfit. One of my buddies is obsessed with it, and it seems expensive for what it is. And its full of dykey women. A lot of the time they seem to be focused on rushing lifts, which is a terrible idea. You need to lift in your own time, at your own pace. Rushing a lift seems like a short cut to poor form and injury.

Boxing is great - I did it at college. It's quite educational and it was just guys.

If you say 'I do not really like to be forced to follow a program' then I have bad news for you..................

Compare yourself to your past self, not to others. Regulars will also respect you for your efforts, when you stick around.

Do you think crossfit is a good way to build global strength ?

Yes. Personally not a big fan, but you can get solid results from that too.

I had the impression that taking more food is for bodybuilding.

This is a misconception, along with things like believing in different body types and that different sports make you look a specific way. They dont.

You have to eat more to gain size however you train / want to look.

Let us know how you rate the book.

Here are some reasons why they are more efficient with your time:

1) Despite how intimidating they look, they are very simple to use. With a few basic compound movements such as bench press, shoulder press, rows, deadlifts and squats, you can train your whole body, forever.

2) Easier and finer progression. When you progress you just add weight, there is no need to learn a totally new move and deal with rep ranges. There is less fucking around. You can lift more for 5 reps? Just add more weight.

3) I challenge you to find a calisthenics leg movement that can duplicate a 315# deadlift, or a 225# squat, which you can achieve within a few months with good diet.

4) Iron doesn't lie. Are you stalling or regressing? Barbells won't forgive you for having a shit diet, they will own your ass.

Do learn some calisthenics for accessory work. Also, absolutely incorporate pull ups and chin ups as main lifts along the big five mentioned above. I recommend you buy yourself some rings at some point if you want to incorporate more calisthenics in your workouts.