Don't gossip or talk trash about your roommate, even if they're awful. Talk to them about it. And when necessary, have a neutral third party mediate disagreements. Having the person you live with find out you've been talking trash about them behind their back ... makes home awkward. You can really trust me on this one.
Don't get your feelings hurt if your roommate doesn't want to be your BFF. Being roommates doesn't automatically mean being friends. And being roommates with your friend doesn't mean they're always going to want to include you in everything. Some people are college roommates and lifelong besties. Some people are roommates and then never speak again. Neither are wrong.