How can you shower with a girl?

first of all, you should be going out with her. And create a trust with her. And be nice and polite about it, make sure she is comfortable with it - don't force her into it, that's a turn off. Mention it in a casual conversation and see what she says about it, and if she seems to have a bad reaction, than you should probably wait a while before trying again. Or just get her into the mood.