First, go to your town or county's building dept. and get a permit, or at least find out what they require to be able to get a permit. This way, you won't get caught half way through and have to rip it half apart and pay a penalty. Chances are you are going to need to hire an electrician to be safe and to be in compliance.
Floor: I have never converted a garage but here are at least some issues you will need to deal with: The slab is uninsulated and it is also sloped significantly (probably). Uninsulated slabs can be cold in the winter and cold and wet (clammy and slippery) in the summer and either way you will lose heating and cooling through it and be uncomfortable. Also, most garage slab floors are sloped a fair amount to drain any water or fuel to the outside. I would put down pressure treated 2x4 sleepers, ripped to accomodate the slope and fill in between with the pink or blue foam sheet insulation and put 3/4" t&G plywood underlayment as a floor. Other guys may know of a way to pour a lightweight concrete product over the slab floor that will level it and insulate it.
Heat: You didn't say whether you had hot water or hot air heat. You could try extending the existing system out there but you would at least want shutoffs in case you wanted to turn the heat or cooling off in there sometimes. If your system isn't quite up to it, or if you might have difficulties in keeping the new room the same temp as the rest of the house then you would want to look into creating a separate zone for the room. For hot air you could use a trolatemp system and there are zoning systems for hot water as well. This way you have a separate thermostat for that room. You also need to decide where your heating runs etc. are going to be, ie: in attic or in floor?
Garage Door Wall etc.: I would remove the door and track and frame a proper wall. If yo don't then you need a wall inside it with insulation and vapor barrier inside. You can make the room easy to heat and cool by putting a lot of insulation in the ceiling. You would need to address the venting issue above it so that the attic is properly ventilated.
Misc.: I would want some sort of a barrier, or at least a curb so that if someone hits the gas instead of the brake in your driveway that they don't run right into the house and kill someone.