Avoid fights. They're hardly ever worth it. Anytime you start to make a stand on something, ask yourself: "Is this the hill I want to die on?"
Also, most of the time, you can just avoid arguing by letting her say/complain about whatever she wants (pretend to listen a little though), then continue doing whatever the hell you wanted to do.
At this point in your life, neither one of you's going to change much, so the confrontation is pretty pointless on most issues. She's gonna spend too much money and get on your nerves by constantly telling you to do stuff. Know that going in and don't get pissed about it.