I would like to offer an alternative point of view: if you have 3 months to kick-start yourself, start by exercising in a very focused way, on something you don't expect to become routine: for example, if you do 3 months of lifting/strength training/high intensity interval training, you will build up enough muscles that making exercise routing when you go to college will be much easier than doing it now.
This September I'll start college aiming for a computer science degree, and I want to use the summer for self-improvement. I'm very uncertain about what skills I should try to learn, though, and recommendations would help.