The best way to work out your size is to try on a pair of skates and see what feels right. If you don't have any friendly team mates with the same size feet as you, then here are some sizing charts to help you work it out.
I highly recommend measuring your feet as accurately as possible. It is no good going up half a size "just to be safe", as you want your skates to fit snugly to allow for the best response from your skate boot. Remember that skates will also stretch up to half a size.
Have a look at the charts below and if you are finding it hard to decipher them, send me an email at firstname.lastname@example.org and I will help you work it out.
Riedell Sizing Chart