Just to start, lusikkamage (aka moosader) has a good map editor, it's open-source aswell, so you can see what she did there, plus she's quite active on this forum, her website is here
http://moosader.com/app-moose-tiles.html
As for making your own editor, just think about what you would do in the editor, a few actions that come to mind for me are
- clicking to place tiles
- selecting tiles
- outputting the map to a file
- reading in maps
have a bit of a brainstorming session, and think about what you would find in a good editor, write down what you think of, and then break that down into logical steps, then it should hopefully become clear
PS: I've made my own editor in the past, so i'm not completely clueless, if you'd like to see what I did with my editor/ game, heres the link to the video, I tried best to explain what the editor can do, and it might give you some pointers as to what is usually included in a (very basic I must stress) map editor.
http://www.youtube.com/watch?v=1xE9gM54mkI
(theres also a slightly more advanced editor I wrote a few months later, the video is on my channel)
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I think I can program pretty well, it's my compiler that needs convincing!
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And now a joke to lighten to mood :D
I wander what programming language anakin skywalker used to program C3-PO's AI back on tatooine? my guess is Jawa :P