what are you making anyway, 2d tile based?

The principle behind a level editor is to database your assets, place instances of those assets in a level, and provide scripting to perform logic. To write scripts, you must first construct the API for your game, which means you have to design your game first. Know what the inputs and outputs are and make them accessible by scripts. For example, read/write character health from a script, trigger an explosion effect, trigger a level modification effect (translate / rotate walls for instance), or trigger a cinematic sequence. These all constitute features of your underlying game engine that you must design first.

I would recommend you take a look at Blender 3D. It's a general purpose 3D modelling tool, but can also be used as a level editor and more. As a standalone, you can use it directly with python (a scripting language) to create Blender games or like me just write a python script to export the data to a specific game file format that your engine will load. Other reasons for using Blender and reading up on its python API will give you a better idea how to organize your own game assets and incorporate scripting to perform logic. Also take a look at the Collada format since it's organized in a similar way, but note that this is only a file format, not a system for executing games. Using Blender or similar tools as the basis for your editor will save you a lot of time from having to deal with the basics of writing your own level editor, which is very time consuming to develop.