Yo, admiral, you need to pick a game to make the editor for.
If you are making it for your own game then you need to make sure you know how to do a couple of things.
You need to have a system in place that handles victory/loss conditions and all the variables that control your vehicles and weapons.
You need to know how to read a file off the hard disk and get your program to interpret the data into something it can understand. You might want to look at making a system to read an .ini file so the player can set up controls. Stuff like that is always useful and it will form the cornerstone for a more sophisticated file format that will hold mission data.
Play some games and make a note of how thigs are set up - and try and work out what information you will have to put in a mission file in order to recreate a similar thing in your program.
As usual it's more about planning than programming, but it's worth it.
Ending a sentence with a French word is so passé
