Well, I guess I'd start by carefully planning.
Like,
How will I manage my object list?
What should the user be able to do with the objects? (Position, Size, Texture, ...)
How will the user be able to alter those things? (Mouse gestures, sliders, ...)
In what format will my world will be exported?
...
The biggest hurdle would be
How will I code this?. And unfortunately, this is only something you can do if you have the required knowledge and experience.
If you finally managed to load an object in DarkBasic yesterday, chances are high that you won't be able to code a full-fledged level editor yet.
Tree models can be made in any modeling editor, or by using special software like TreeMagic (You can buy this product on the TGC website).
