download rom manager from the market (if you haven't already), then select the option to flash clockwork recovery, (you may also be prompted for superuser access, allow it/remember the choice) do it about 5-6 times. then choose the option "reboot into recovery". In there you will see the option to wipe data. use the volume keys to highlight it and press down on the trackball to select it, you will then be prompted to confirm your selection. While in recovery, you may also want to wipe the cache, and dalvik (in the advanced menu), the first boot will take a while after performing those steps (up to 5-6 minutes).
edit: on another read of your post, you can't boot into your phone at all?
if that's the case, press and hold down the volume *down* button until the phone turns on, use volume keys to highlight recovery and use the power button to choose it, then follow instructions above, and reflash rom, or restore nandroid if applicable.