You'll also benefit from having a graphics program better than paint for making and modifying textures. Many people use Photoshop (expensive), Paint Shop Pro (cheaper - older versions are good enough), or GIMP (freeware on the web).
Good luck - GMax takes a bit of learning, but once you crack it it's well worth it. Stream engines are about the hardest thing to create - it's probably best to cut your teeth on some buildings first and work up.
Paul