Hello, I Am About To Embark On My Procedure Of

Hello, I am about to embark on my procedure of starting and taking lithium for at least half a year. I noticed that one of the side effects were weight gain. I have taken several other drugs including Geodon, Citalopram, and another one I can not remember. I have gained quite a bit, about 15-20 pounds. I have been exercising and eating different. However, sleepiness is a whole different thing and I know I won't have much time or energy to exercise especially with school coming up.
Do you have any tips or recommendations on reducing the effects of weight gain or even weight loss whilst I am in this temporary setback? Such as diet specifically? I've been fatigued from previous medicines and at those times I had barely any energy and definitely not enough to exercise for long enough, it wasn't until weeks after I stopped till i got my full energy. I am worried about my weight because I reached the overweight scale and I want to do what I can to avoid gaining extra weight maybe even losing some weight while I adjust to the medications.
Please help, thank you!