I''m making a 3D game using opengl, and I have no clue on how to begin making a map editor. I just decided to pickup opengl programming, I enjoyed sprites I can make a 2D isometric, etc, etc, map editor but not this. I want to use a lot of terrain so keep that in mind if you know of some tutorials. Thanks, Sorry for the crudely written post :}

You could look at the BattleZone (PC - Version ;-) ) map editor. It is published together ,but a little bit hidden, with the game. I think its in the "edit" directory of the game and there is a readme file which discribes the use of the editor.
I liked the interface of this editor because you were able to make smooth landscapes. Perhaps you find a discription of the output file format. Then you could reload it into your level editor ...

I''m doing exacrtly rthe same thing.But what I used for a level editor was worldcraft.Someone wrote an article on exactly this kind of issue on www.Flipcode.com The article was entitled "Level editing" or something...look over there. It''s a good place to start.