If you go to the links on the main page, Public Domain Superheroes Wiki or the site in my signature, The Free Universe, you will find info on tons of PD characters that you can use. Probably the most notable ones you'll recognize include the original versions of several DC characters including the original Blue Beetle, Plastic Man, Captain Marvel, Phantom Lady, Uncle Sam and many others. But there are also some more obscure characters which are equally as awesome. We are also creating freely licensed new characters on these boards if you wish to join the discussion!
I enjoy microheroes as well (I'm a member of the International boards). Like Bard said...only way to get better is practice...and we will love you on these boards if you use public domain or freely licensed characters as your subject matter!