I just started getting interested in USACO, but I don’t know how to prepare for it. Do you just do a bunch of problems or are there books that you guys read?
Your USACO score comes down to your ability to solve the problems during the contest, so solving problems is obviously the best way to improve 
. As @dongliu0426 linked (above), go on USACO Guide and read through each module. The “general” section should provide more information on how to start as well as how to practice.