Do a search there are some threads on this.
It does feel like you have to pee, but empty your bladder before and you won't have anything to worry about, it's not pee that comes out.
For some it takes a bit of practice to get there and you just really have to relax.
Pick up a couple of books. 'Unleashing Her G-Spot Orgasm : A Simple, Step-by-Step Guide to Achieving Ultimate Sexual Ecstasy' and 'Female Ejaculation and the G-Spot'.
Read them with your BF and have fun.