Level editor normals

Hi! I''m planning to do a level editor for my next game (the both editor and the game are still in design-stage, though I believe that I could program a simple editor which allows users to construct levels from triangles/quads. But I have no idea how can I compute proper normals for a triangle/quad of any shape? (the normals are used of course for lighting) I must say that the levels are not a terrains or other "grid" type stuff, they''re small levels constructed from triangles/quads floating in the air. And if somebody could please answer, please explain in detail --- Mika Halttunen