First of all, read this wikipedia article about rom hacking so you know what you're dealing with;
http://en.wikipedia.org/wiki/ROM_hacking
Next, see what kind of tutorials we have about graphics editing in hax.iimarck.us. By searching, you should find out that tilesets are either in de-compressed or compressed format. If a tileset is decompressed, you can use a tile layer editor for editing them (like explained in that wikipedia article). If they're compressed, you need to insert a new tileset yourself that you compress to the rom file for it to be displayed correctly. For this we have a tool called AgiXp (like you should find out by checking hax.iimarck.us).
http://en.wikipedia.org/wiki/ROM_hacking
Next, see what kind of tutorials we have about graphics editing in hax.iimarck.us. By searching, you should find out that tilesets are either in de-compressed or compressed format. If a tileset is decompressed, you can use a tile layer editor for editing them (like explained in that wikipedia article). If they're compressed, you need to insert a new tileset yourself that you compress to the rom file for it to be displayed correctly. For this we have a tool called AgiXp (like you should find out by checking hax.iimarck.us).