my boyfriend proposed but confused about muslim marriage?

[Edith A]

So my turkish boyfriend proposed to me yesterday . My boyfriend and I live in NYC and his parents live in Turkey. His father came from Turkey and along with my boyfriend ask my parents permission for marriage. My parents accepted. I have to say that I was very impressed by the formality of my boyfriend and his dad. But here is my questions, after the engagement, my boyfriend and I agreed to marry in a year or two from now, (we are both 25) however, he insists that we need to get married by an "imam" before we get marry in a civil wedding. The reason is because he doesn't want to commit a "sin" by having sex before marriage ( we know each other for 8 months?

Is there anything that I should know about an islam marriage perform by an imam. I don't want to be legally marry at the moment, so I want to make sure that If I do this I will still be legally "single".

Your answers are greatly appreciated.
PS-I'm Argentinian.
i forgot to ask, Do I have to wear a white dress or dress a certain way?

My boyfriend says its not necessary for my parents to go, since it can be just us two? is this normal?

and about the dowry! wow! I had no idea I was suppose to get a gift..he he....what am i suppose to ask for?
also, we have had premarital sex, but my boyfriend is now feeling that we should do it the "right" way.