This has been a constant question every since the fair sex came into existence. I’m sure you will think of something. Every other person who got wed has done the same thing you will do. Just say “hi”.
I was always told it doesn’t matter what people say in these situations as long as you say something. Clearly indicating friendship/interest, whatever, is the important thing that you need to do. Just prepare some dumb comment (or a few of them) about the weather or the priest or how crowded/empty it is. Whatever. Then move to standard topics: What people do for a living, study at school, how long you have been at the Church. Holiday plans.
I have been in this exact situation, do you know what I did? I waited until Sunday after Mass, went outside and introduce myself, found out that she was a student from Colorado, and we became friends. And if you think I was nervous talking to her, you are absolutely right.
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