first off, make sure your theme is zipped with with only one folder inside it, and that your theme includes a screenshot.png file.
also, check out this:
once you're sure it's ready, go to
and upload and tag. make sure to ask theundersigned for a login so you can update your theme as necessary.
if it supports widgets, email it to andy:
widgets [at] wordpress [dot] com
and if you're a deviantart.com kinda person, upload it there and tag it 'wordpress'