Capitalize after the colon if there are a secession of sentences following. If there is only one sentence following, do not capitalize. That's the general rule, but it can change depending on the sort of style guide one is using. There is the Gregg, APA, Chicago, AP and whatever else is out there that you can choose from. Gregg style says always, always, always capitalize after a colon.

One thing all people need to remember, especially on this board, is there are different rules of writing. You are not right and neither is the next guy.