I agree with noisebeam on the adjustments to your mountain bike. While you have it, you might as well set it up the best you can and use it. Check out www.sheldonbrown.com for info on bikes (hopefully some of the other forum members will be able to point you to training sites).

Getting involved with a local cycling club is a good idea. They could probably help you find a new or used road bike at a good price and they could help you set goals for training.