Depends on what you're aiming for. If you're wanting to render some more 3d terrain then something like Terragen or Blender or some other 3d program would be the way to go. If you want to take a grayscale image and just turn into a basic 2 color map (just as a starting point -- black ocean, white land) then any raster program like Photoshop, Gimp, Paint Shop Pro, etc could handle that. Each of those programs has a different way to get there in terms of which buttons you click but the process is basically using a threshold adjustment or a brightness/contrast adjustment. Once you have your land separated from your water then you can start putting down colors and symbols and text. If you have Photoshop I can go into this further if you'd like, if Gimp then one of the guys who specialize in that can help further. So the only thing left is which programs will you be using and can you post the grayscale?