Once you have made the door, you need to UV map it in the texture editor. This essentially involves laying out your model flat as if you were unwrapping it, but first you need maya to project your model. Go to edit polygons>texture>...and then choose a projection method. For the door, I might pick planar or automatic mapping. Once you are in the texture editor, back click to choose UVs and then start arranging the various parts. Once finished, take a UV snapshot which you can then open in Photoshop to paint. Sorry this is a bit vague - it's new to me too and I cannot check my computer at the moment - just do a search on this site and there should be plenty of threads explaining the basic technique.