Really, when it comes to an omelet, there's not much to consider in the way of ingredients, since you'll just be adding whatever you like. Really, it's hard to add something to an omelet that ruins it.
No, the most important thing is how you cook your eggs. Make sure that they are properly stirred before you try and make your omelet, flip only when necessary, yada yada. I prefer to add a small amount of butter to my eggs, but that's just me.
In terms of fixin's, you can't go wrong with small chunks of bacon or grilled chicken for meat. Grilled green peppers, onions, or tofu also go well in an omelet. One thing to keep in mind, though, if you plan on adding a bunch of stuff to it and are new at omelets, you might want to make it an egg wrap instead.