Hi Zdravko,
I too weigh 55kg and whilst I'm not trying to increase you muscle size, I can certainly help you with some tips to do it.
Firstly, you are going to have to take up some sort of resistance training. Whether that be at a public gym, school/uni gym or at home with your own equipment (if you decide to buy it) is entirely up to you. That decision can influence the specific exercises you can do, so I'll try to keep it as generic as possible, using free weights.
Two great exercises have already been mentioned, with good reason. The squat and deadlift are awesome and certainly do help with overall muscle mass, as well as in the specific muscles they use the most. I will list a general exercise program you can try, with links to demonstrations of each one. Then I will let you know some principles to follow and extra information. You must still see a gym instructor or alike to get shown the correct technique for each exercise, otherwise you are putting yourself at risk.
Monday -- Exercise -- Sets -- Repetitions Per Set
Barbell (BB) Squats 3x12
BB Stiff Legged Deadlifts 3x12
BB Lunges 3x12
BB Standing Calf raises 3x15
Wednesday
BB Close Grip Bench Press 3x12
BB Upright Rows 3x12
BB Bench Press 3x12
Dumbbell (DB) Supine Triceps Extension 3x12
Friday
BB Deadlift 3x12
Wide-Grip Pronated Chin ups 3x8
BB Bent over row 3x12
BB Standing Bicep Curls 3x12
That is not a definitive program, there are a million and one possible variations. It should be a good start. I used the one site for many of the links, it was far quicker than finding the best for each and that site gets high pageranks, overall it's very good. The days you workout are your choice, that is just a guide because having a day rest between session is a good idea at this stage.
To work out the weight you need for each exercise, find something where you can do 12 reps comfortably enough but with quite an effort in first set, rest 1 minute, do another 12, rest 1 minute, then find the final 12 hard so you might only manage 10. When you can do 15 reps in the 3rd set of an exercise, it's time to make a small increase in weight.
In this case, 1 minute is always the rest to have between sets if you can. 2 minutes could be ok, but 1 is better. During each set, try to make the repetition nice and controlled, moving the weight slowly both ways. This helps maximise the muscle building effectiveness and is different to strength training.
On the nutrition side of things, half an hour before a workout have something like one of the following:
- A glass of Milo with milk
- A tub of yoghurt
- A tuna or chicken sandwich
- A banana or apple and a glass or milk
A similar snack within about half an hour after your workout is also good.
Your general meals should basically be well balanced, including rices, pastas, potatoes, red meat, chicken, fish, green veggies, nuts, all other veggies. Try to split your daily food into 5-6 meals, rather than just 3.
It's also important to get enough energy overall throughout the day. In fact, you want to be a little bit over your requirements. To help you work out how much energy you need, the following link is the best I've found of its kind.
Energy Calculator - Daily Calculator
I have probably left lots out, but that is probably long enough for now. Hope you find it useful :-)