This has been a pretty mixed bag of responses for the OT. Some people are bang on, but the amount of misinformation here is boggling.
OT: Wikipedia is your friend - that's your first start. Get a general overview of Islam and what it means to be a Muslim.
I'm coming up to 3 years of dating a Turkish girl (and have lived in Turkey for the past year), so know her and her family pretty well. I've come up against a lot of bullshit, quite frankly.
I could just lay out all my advice here, but quite honestly I'm not in the mood to bang out a 5000 word essay on my keyboard for you; because that's what it would take.
All people who identify themselves with a religion label come in a scale of 1 (it's just a label and they kinda enjoy the holidays) to 10 (they're full on crazies).
When I met my girlfriend, she was kinda hovering at around 6-7 (though from the way she spoke I THOUGHT she was more of a 3-4). 3 years on and she's like a 1-2 on the scale now. I've not talked her out of her religion, just through general discussions (and her seeing my home country, the UK) she's kinda let her mind open much more than it was before she met me and is seeing the contradictions and bullshit that she was actively ignorant too before.
So... where does your girl sit on that scale? More importantly, where do her family sit?
I've had to overcome a lot of prejudice from certain elements of my girlfriend's family. Her grandparents (who, admittedly, love me to bits as a human being) just wish I was a Muslim, because if I marry their granddaughter as is, I'm committing - at a minimum - my girlfriend to hell, possibly even the rest of the family.
Her dad - again, really likes me as a human - has given every all sorts of trouble over my non-Muslim status.
These people are pretty mild and "modern". If your girl's family are worse, just really, take my advice and leave well alone, it's not worth the trouble.