The best you can get for simple changes (maps, encounters, items, spells, etc) is FFHackster... look it up on google or something... you'll find it quick...

Other than that there's not much you can do, unless you know how to hex edit... but even then theres not much you can accomplish before corrupting the rom. The problem with editing the rom is that you can't do anything other than the things i listed above, because you cant go over the original size (i believe FF1 is like 128 or something), and you can't go over the character limit (ie. making an item name longer, requires shortening another).

If you want to make an edited FF1... get mihilas' RM2K3 source code for FF1 and edit that in rm2k3. Though I'm not sure if he locked it or not.