Uncle Moo Moo,
You're 15+ years old and you're already 1.75 m tall?? Good grief,
what do you guys eat nowadays?? Long beans?
Anyway, I don't know what gyms are charging, but most will not
allow anyone under 16 to be a member.
Er....if you want to put on weight, skip the cardio (running) for
the moment. Put on a bit of weight first, then start your cardio.
At your age, you just need 2 weeks to get your stamina up from
0-100% lah, don't worry.
55 kg is a bit light for 175 cm. At this stage, I don't think its
important to watch your diet like a professional does yet....you
probably could eat anything without getting into trouble. The
important thing is just to eat much more than what you're eating
now. But stick to white meats, egg whites and tofu....and some
fibre (fruits, vegetables) and carbo (rice, white bread).
Remember that one's weight is actually a function of energy (food
energy, calories) input versus output.
Generally,
When input > output, weight gain can be expected.
When input < output, weight loss can be expected.
When input = output, weight is consistent.
To put on weight, you need to input more energy (ie. more food).
And not increase your output too much.