Just start asking her and then the rest will flow out. Take deep breaths and try todo it with a close friend near by for moral support... but don't have him stand awkwardly by when you ask her. See if he'll stand a few feet away and not stare at you when you do it.
It's totally normal, we've all been there. People will tell you to relax, but that's not possible. Just be confident, she's lucky you're talking to her (not really, just pretend!) haha
*I resolve to call her up, a thousand times a day, and ask her if she'll marry me, some old fashioned way, but my silent fears have gripped me, long before I reach the phone, long before my tongue has tripped me, must I always be alone?*
Develop the friendship first!!! Then say, " you know I'm no good at this game stuff, I want to ask you out but I guess I'm just too weirded out about what you'll say". Be real. She will appreciate the real you not the player you think you need to be. Go for it