You're already in a big hole because you're thinking about it so much, and that is going to come across so strongly when you approach her... there isn't anything that I can tell you do to do that is gonna work because you need to experience how you go about this sorta thing with a girl before you can become confident about it and be able to do things.
What I suggest you do before you speak to her and before you ask her (which you should definitely do - you're never gonna grow if you're too much of a ****, which is easy for me to say on the net but meh) is close your eyes that morning, think to yourself yep i'm definitely going to take her aside and ask her TODAY no matter what, it's happening today, and then close your eyes and imagine you're with her and she says yes and how bloody good that will feel... then go into it with the confidence that it's already happened in your imagination so that's how you're gonna feel when it actually DOES happen.
If you keep yr cool and stay a bit of a smart arse and don't be too nervous then you'll be all sweet dude! Don't think about it too much, and definitely don't tell her that you 'post' on MyWii or any of that **** straight up, just don't even talk about it yet heh.
Good luck (Y)