I have been enrolled in PF for quite some time, but I just recently became active in the community. I think in addition to the great outline that pennfosterstacey provided above, the best way to get your feet wet in the community is to poke around, explore a little bit.
I feel like each time I log in, I find something new! Definitely view the academic groups to connect with other students in your same program. The social forum is also a great resource in getting to know other students. There's no better way to learn how to navigate, within the community, than to jump in head first and let your clicks take you wherever they may Have fun!
Amy