Girls HELP Please! How should I tell her or what should I do?

So theirs this girl that I like and well I'm sure she likes me as well because she's always flirting with me, calling me loser and always trying to get my attention or I catch her looking for me but anyways I don't know what to do, I wanna tell her that I like her but how? I'm a strait forward guy and it's just bugging me that I haven't told how I felt about her, I don't like to hide how I feel anymore. I don't have her number but I have her as a friend on facebook. I only see her Tuesday and thursday at marine training but there's not much time to talk. I was suppose to ask her for her number but things kinda got ruin because I had to handle paper work for the marines but anyways I don't want to get her number of Facebook because it makes me look pathetic I like to get in person. How should I tell her?