2D Map Editor

Hi guys! I wonder if anyone would very kindly help me with my 2D Map Editor that I'm making? Basically it loads in an XML file for the graphics that it uses and an XML file for the map. After the user has loaded both these XML files it will render the 2D Map and also the graphics that can be used for editing on the map. Ok this is now where I'm stuck. I'm not sure how to do tile picking, like how to select a tile that I can then paint on the map? Also I need help on how to save an XML file aswell. This is what I've done so far:

Download Map_Editor_C_sharp.rar for free on Filesonic.com

I've included everything thats needed. The Graphics.xml and TestMap.xml are the 2 files you need to load when you open the map editor by clicking on file then load and then choosing the appropriate xml file. They're both in the GG_W7 folder. You'll notice also in the GG folder that theres something called GGame. This is the test harness that any Map that I save has to be loaded in to via an xml file. Any help would be really appreciated because I'm very stuck. Feel free to have a stab at it yourself if you like. I'm sure there must be people who've done something like this before.

Thanks

Dan