Hi JessF!
I'm also in the same boat as you. Never had kids, none of my friends have kids, some of them will never have kids, etc. To add insult to injury, I'm the youngest of two and never babysat a day in my life, so kids were (and still are to an extent) completely foreign and bizarro to me.
I live with my BF, who has his 11 year old boy full-time, minus one weekend a month when the kiddo goes to his mum's. I had the benefit of getting to know kiddo first before I moved in (had my own flat when I moved over on a student visa), so I definitely agree with those that suggested it. Just spend the first bit of it getting to know her, maybe taking her somewhere that she likes to go.
In the beginning, I stayed well out of any rows or disciplining, kept it friendly and fun, and got a feel for how BF was raising the kiddo. My role has changed gradually, especially with moving in, but I'm much more comfortable with it than I was.
One thing I kept having to tell myself, when it first came to having to enforce rules or discipline is that just because I'm not his mother doesn't mean I don't have any grounds for keeping him in line when he gets cheeky or telling him when he's not behaving appropriately. It can feel weird at first.
But then I think about teachers, and how they are almost in the same position as a step-parent. Here you are with someone else's kid, playing an active role in his/her life, and sometimes having to tell them no or deal with bad behavior. It helps me to feel a bit more confident about my role and to know that I have some ground to stand on.
Also, don't hesitate to talk to your partner about it. I can't tell you how many times I have called BF or said to him "Hi, I have no idea what I'm doing here, help!". He always jokingly says "You think I do?!", but really it's good to have some support and guidance with the whole thing.
I have to say, despite some tough times, it's really been rewarding for me. Kids just want to have fun and be loved more than anything else, and ultimately, all they're really going to want is to get along with you. They can sense when you feel awkward or distant (seriously, kids are psychic I think), so you just have to try and get past it at first.
It takes time and patience, a few mistakes, but you'll be fine.