Put this in an npc (in the level)
NPC Code:
if (created) addtiledef tileset_era01-main.png,,1;
dont worry if you dont under stand that just now. If you do a forum search for "newbies scrip bible" there should be a thread with a scriping guide that you can download. Thats where i recommend you start.
Read that, ask questions about anything your unsure of.