How to eat for energy?

Hello. First I want to apologize a little, because I'm self diagnosed since my doctor wouldn't take my weight problems seriously (and I didn't have the resources to see a different one) so I'm not sure if my thread belongs here. I'm recovering from some kind of eating disorder that caused me to be underweight, and hate eating. (I will be seeing a nutritionist at my school soon, but I don't know if they will help yet).

I would like some advice on what to eat to allow me to work out in a healthy way, and I think this might be a helpful conversation for other members too. My goal isn't to lose weight, but to build up leg muscles to I can go bike riding with my friends. I was hoping to go to the gym today, but I didn't eat much and I'm not sure if it would be bad for me to go. In this situation, is there something I can eat that will give me the energy to exercise safely?

As a background, since I was unhealthy for so long, my 'energy reserves' are low, and if I don't eat for a day, by the late afternoon I'll be overheating too easily and sweating. Then, I'll even get fatigued doing simple things like washing my hair, and exercise will make my heartbeat spike really fast.

Basically, what do you eat for physical energy?