Weight training
This is advice for gaining strength as your priority - but size will come along with it, especially in the first several months.
Train three times a week, with a day off in between. There are two workouts, alternate them.
Workout A
Deadlift
Bench press
Bent over barbell rows
Workout B
Squat
Overhead press
Heaves - aka chinup/pullup, hands towards or away, spread or close, whatever's comfortable and lets you get up there
Ask at your gym for coaching on doing these lifts. If you're using a home gym, ask an experienced friend or get a trainer at least for one session, it's important to do them right. Failing that, look on youtube for "rippetoe", there are some good videos giving coaching for these lifts.
With each exercise apart from heaves, do 1 very light set of 20-30 reps (movements) - just the bar or something. This warms up your muscles and stretches them out ready for action.
Then add weight and do 5 reps. Repeat 4 times. For example, deadlift might look like,
20x bar only (in fact 20kg for an Olympic bar)
5x 25kg (ie bar +5kg)
5x 30kg
5x 35kg
5x 40kg
5x 45kg
Let's say you can't manage 5 reps on the last set, only 3. That's okay. Try again next time. When you can do all 5 sets of 5 reps with good form, add weight to each set. The idea is that the first 5-rep set should have roughly half the weight of the last set.
Between each set, rest for 2-5 minutes.
With the heaves, it's a bit different. If you cannot do even 1 heave, then do "negatives". Jump up into the top position and let yourself slowly down. Rest for a moment. Then jump up again, and let yourself slowly down. Do that 10 times. Next session, 15 reps total. After that, 20. And so on. You're not worrying exactly how many you can do in one go, just how many you can do in one workout - you can spread them throughout your workout if you want to, you don't have to do them in one go.
When you can do 20-30 negatives you should be able to do 2-3 full heaves. Now work on a total of 20 heaves, the next week 30, then 40, and finally 50. When you can do 50 heaves in total, you'll probably be able to do 5-10 heaves in one go.
Work up to doing 3 sets of 10+ heaves.
Try all this for 12 weeks, then have a full week off and reassess, looking at your progress so far and reconsidering your goals.
Fitness and agility training
This is best addressed by your football coach and training, or by other posters here like Markos (PTC). I know just about beginners, and making them stronger, bigger or smaller, and fitter generally.
Diet
This is advice is also based on your wanting to gain strength, and with it a bit of size. You need energy and nutrition to power your workouts and repair your muscles after the workouts.
Eat lots of fresh fruit and vegies, nuts and beans, some meat, fish and/or dairy. If you want to get bigger, eat lots of starchy foods (spuds, rice, pasta, bread).
Make a workout drink out of 500ml milk with 100g skim milk powder and 3 eggs. Drink half before and half after the workout.
If at any time you lack energy, you are probably not eating enough. If after a month you have not put on any weight nor have you got stronger, you are definitely not eating enough.
Make sense?
Go down, make a weekend of it.