To actually answer your question, and give you a place to start, look in RMXP's Database (press F9). There will be a tab called "Tilesets", which displays all the tilesets along with various bits of information for each tile (including passability, priority, terrain tag, etc.).
Have a look at some tutorials and guides to see how tiles in a tileset should best be set up.
Also note that, if you're creating a map and want to erase some tiles, you must use the top left (blank) tile to do so. If you use any other tile, odds are it won't do the job properly. That's why all the blank tiles in the tilesets I made (which come with Essentials nowadays) are filled in with a red [X], to make sure people don't try to use them.