Self-confidence can only START with an attitude change. You need to have something to base the attitude on, in order to really feel confident. So if you can write well, but have trouble speaking in public, practice reading what you've written. In front of a mirror, alone in your bedroom, whatever. Keep it really short at first. Maybe you could ask your best friend to help you role play a little.

One important part of the attitude change is to keep reminding yourself how much better you'll feel after you've been successful. Praise yourself for what you've done right, and be easy on yourself about the things that didn't go so well. We're always harder on ourselves than anyone else is.