Decide if one of you is going to pay all or some of the bills directly (and the other pay her half to the roommate) or if you'll each write a check for your portion of each bill and submit them that way. Whichever you decide, set a schedule for bill paying and stick to it (that includes paying each other if one person is going to pay the bills and let the other roomie reimburse her).
Set housekeeping rules. Who cleans what and how often? Each of you should be responsible for your own room (and bathroom if you have a private one) but common areas need to be addressed.
Overnight company...how will you handle that? Is having a man(assuming you are straight women, woman if gay) overnight acceptable? If overnight company (platonic or not) is okay, is it okay or not for that company to be at your place while the person they are there to see is away? For example, if your roomie brings home a guy for overnight, and she gets up in the morning and goes to work...is it okay if he stays there while she's gone either while you are home or after you leave as well?
Food. Who buys groceries and will it be your food and her food or community food?
Drinking and/or smoking at home. You'll need to agree on that.
Agree to talk about any issues before letting them stew and potentially denting (or worse) your friendship. Depending on your personalities, it may be easy and great fun to live together...or you may find your friendship is more likely to stay intact if you live apart. Being open to communication and open minded if the other needs to talk or vent, will go a long way to helping y'all keep things peaceful though, and having a successful living situation.
Best of luck.