As a new commuter just a few months ago I'll take a stab at some of your questions.

1) Almost any bike will work to get started. Since you bought it just a few years ago I hope that it is sized to fit you comfortably. If you have not ridden it much you might want to take it to an LBS (local bike shop) and have them check things like the seat and handlebar height and position to make sure they are correct for you. Getting these in the correct position goes a long way toward making riding comfortable, preventing knee pain and maximizing efficiency. You or the LBS should also check that everything on the bike is working well and that everything is properly lubed. Obviously a mountain bike will not be as fast as a road bike, but it will certainly give you a good workout.

2) As to websites use the search function on this website or specific forums to find a lot of information. You can also check out Sheldon Brown's site for good inforamtion on riding technique and strategies. Just remember to read all web postings with a grain of salt -- there is much good advice to be had but also many old wives tales, urban legends and just BS to be had as well. As you get into it you will have more questions which you can first start to research here and then branch out to the web. Also your local LBS might have information and advice that will fit your particular area and conditions.

Take it easy as you get started and generally yield to your body's signals. I hope that helps and gets you started on the right pedal.