How do i code a level editor?

How do you go about designing and coding a level editor for a 2d game?

I want to make a level that has walls and if the character hits the wall then he doesnt go through it. The wall will be a bitmap image, say 40,10 pixels. I dont want to use the 'imagecollide' function as when i add more gfx to the level it would make the code sloppy.

I saw examples of 'data' and 'read' commands but cannot find a clear tutorial regarding this and how to implement it into a game

thanks for reading
graham