Thanks, congrats on getting in! Your situation is similar to mine - I'm OOS and had no idea what to do about housing. Luckily I had a relative in PR who is a realtor, and she found a 3-bedroom apartment for me. I signed the contract over email and then I went on the FB page and advertised my apartment to other first years, and that's how I got my roommates.
Most of the first years live in an area called Los Prados (the most popular complexes are Armonia and Serenna), and some live in Caguas tower.
I recommend that you first check the Facebook page and see if anyone is looking for roommates or renting out their apartment/house. Last year there were some students in the class above me that rented out their room just for the summer, which could be a good option if you want a temporary place to live during your summer classes while you are searching for a long-term apartment.
If you want to find a long term lease now, you can go to
Real Estate Puerto Rico ,Bienes Raices en Puerto Rico Clasificados Alquiler Online and search for apartments by area. The listings usually have a phone number you can call for more information.