Each tile is 32x32, and a tileset has to be/can only be 8 tiles wide. If I were you, I'd follow the general layout of the preset tilesets. There's also templates for tilesets, autotiles, and character sets on the forums if you don't feel like making your own. Also, once you make/get your template, if you feel like taking things from other sets you can either use mspaint (most user-compatible, but time consuming) or GIMP (most user friendly, but you have to have a decent processor to use it). All of this info is already on the forums, so you shouldn't have any difficulties.
