First, sketch out your ideas and write notes: Plot, Characters, Abilities, Enemies, Powerups, Locations, styles, etc.
Then, I would start by deciding how the game will be played that you will be making. (2d/2.5D/3D : Platform/FPS/RPG/RTS, etc.)
Then, make a simple level and maybe a few objects that will be in the game with blender or 3dsmax or photoshop, or notepad, or etc.
(Make sure to take notes on what and how you created these things.)
Now, work on creating a basic game engine for that style (moving, sliding, jumping, shooting, etc.)(again, notes)
Then test -> debug -> test -> debug -> test -> debug -> etc.-> etc.
Quote: "most people program their own level editors which has features specific to their game."
THEN, start working on a level editor. (Taking your design notes into consideration.) It sure would suck to make an editor without knowing what you will be using it for and what objects you will be using and then have to redo the whole thing because it doesn't have the functionality you would have liked.
Then test -> debug -> test -> debug -> test -> debug -> etc.-> etc.
Anyways, just my two pence.
There's something in this room that makes you can't speak well.