Are you seeking to create a tilemap based level editor or polygonal one? (Also I assume you are aiming at a 2d game)
Both cases usually have different representation of the game world. With a tilemap, a 2D matrix specifying which tile to use at each (X,Y) is the most logical.
A polygonal map representation, at it's most naive form, will store an array of polygons. The representation of the game world greatly affects how you would implement
the level editing area in your editor