congratulations on your acceptance. you'll really like pcom. first trimester is going to be anatomy, histo, development, and radiology. if you've never taken anatomy before, i recomend getting a netter atlas and at least familiarizing yourself with the bones and bony landmarks. it'll really help orient you in your first few weeks of anatomy. we start with the back, shoulder, work our way down the arm, then move on to head, neck, thorax, and down to the feet. if you want to do more than bones, start familiarinzing yourself with muscles, atachments, innervations, and actions. there are plenty of online anatomy resources on the web, www.medicalstudent.com has a bunch, so don't run out and buy a text book yet, chances are you'll never use it, but you will definitely use the netter atlas, so you might as well get that now.
if you have taken Histology before, and got a B or higher, there is a chance to take a histology lab placeout exam the first week of orientation. this will give you a precious few extra hours each week, and its totally worth it to study for it... but they won't let you test out unless you've taken college histo. if you have, PM me, and I'll give you further info about how to study.
Keep in mind, plenty of people do just fine in med school without any preparation, so don't waste any good opportunities to slack off before you start. but it is nice to start out a little ahead.