To find secret levels at the start of the game immediately go left to the desert, then continue left to the fire zone. Head to the uppermost left corner of this area and you should see lava with a patch on it that looks like a section of the Firewall spell. As quickly as you can, walk on this patch and you'll be whisked away to secret levels.

When you reach a new unexplored plane, take out the disk and kill the druid. Instead of returning to the village, you will be returned to the plane you died on. Insert the disk, and start playing again, use this as many times as you want. However the problem with doing this is that you lose all the spells that you had before, and start the game with the normal complement of spells. So you may not last long on later levels, but you can at least explore them.