Here is a great selection of articles targeting new people to the hobby. If you accumulate some basic knowledge now you'll save yourself a lot of trouble later.
http://www.canreef.com/vbulletin/forumdisplay.php?f=51
A 20gal is typically 24"x12"x16" LxWxH
Research every purchase, this includes equipment and livestock.
A lot of money can be saved by buying used equipment instead of new.