Can I pick my own roommate?
Yes! You can request a roommate on the housing application that you know from high school or chatted with on the Facebook page or met through some other event. You can also pick to have a random roommate which many people do so don't feel pressured to find somebody. It honestly can work out great or not so great with either option and ultimately this person is your roommate not a required best friend and what matters is that you can live together peacefully.
Can I pick which dorm I get?
No. Where you get placed is random.
Which is the best dorm?
In my opinion, new dorms are the best, but I am incredibly biased since that's where I lived my first year. All joking aside, your living situation is what you make of it. If you put yourself out there and get to know your dormmates and hallmates you will be fine regardless of where exactly you're living.
And now on to the actual dorms!