To record from a more modern game like SA2 or the recent Sonic the Hedgehog title, you'll need either 1) a VCR and a willingness to send a tape by mail to someone who can convert it to a computer-readable format; 2) a DVD recorder and a drive on your computer capable of at least reading DVDs; or 3) a TV tuner card for your computer so that you can capture composite input directly to a video stream (if you use choice 1, you'll send the tape to someone who's capable of choice 3).
Old games for systems with comparatively slow processors, such as the Genesis games, can be emulated fairly easily. If you have the original Genesis and games you can use one of the above options, or you can run the game in an emulator and either use a separate screen-capture program to grab the window, or you can use a modified version of Gens at
http://upthorn.mspencer.net/gens_movie_9Z.zip which has an option in the TAS Tools menu whereby the emulator records its own output to an AVI file.
In order to submit a video you've made, you can go to the left-hand bar and click the Videos link that's under Submissions (not under Media). Make sure you've compressed it to a sane size (probably 2-6MB per minute), then point to the file from your hard drive on the upload form.
