Lot's of good advice on these boards, but start by riding a lot as in five or six days per week if not every day. Make some rides longer, some faster, and some shorter and easier on days when you don't feel sharp or need some rest. But basically ride a lot.
If you've already been doing that then the next step is focused harder but not gut busting efforts so several hours where you try to roll as much Tempo as you where you notice your breathing and you're making a good effort but it's your own best 'fun and fast' pace and something you can hold for mile after mile. Then a couple of days a week pick it up further with some 2x20 style Threshold work. These are mini time trials at least 10 and better yet 15, 20, 30 minutes long each or longer where you ride near your limit for that duration but backed off enough you can actually do more than one and do it in normal training without burying yourself mentally or physically.
Do that kind of stuff and keep riding a lot, find local shop rides or other group rides you can join to work on pack riding skills and you'll be ready come fall to ride with the collegiate team and to build from there.
But basically to build your foundation of cycling fitness it starts by getting on the bike a lot and sustaining faster speeds and more efforts on some days with an emphasis on 'sustain' not just short gut busting sprints.
Good luck,
-Dave