If you're finding you don't know how to ask your friend out, just walk up and do it without thinking. That'll stop the nerves kicking in and tying you up, and get the whole thing out in the open. Whatever happens, you'll feel relieved and who knows - you might surprise yourself
Walk up and say "hi, fancy going for a coffee / drink?" and in your head you'll go 'holy fuck, did I just do that?!' heh.
Oh, and if you want a slap, try saying: "fancy going halves on a bastard?"
hehehe.