Pay attention to behavior that isn't respectful of you. And remember, even if you don't succeed in getting out of a date rape, it's not your fault. Decide what you want. And remember, your girlfriend will be making these decisions too. Talk to each other.
Communication is the key to a good relationship.
Tell your girlfriend what your limits are. Find out how she feels about your decision and what her wishes are. Listen to your partner. Trust your feelings. Trust the very first messages your girlfriend gives you.