First, press F2 on your keyboard. Then go into your .minecraft folder. To get to that folder, depending on the OS, go to run, type %appdata% and then click .minecraft. If you are on a Mac, go to Library, Application Support and click on the Minecraft folder. Once in the minecraft folder, open the screenshots folder and find your screenshot.
To post them, go to an image hosting site such as:
www.imgur.com-or-
www.photobucket.comThen choose the correct screen shot and post the [IMG] code into the post.