Best way to build floor above blacktop
To deal with just the blacktop you basically need to jackhammer up some holes in the floor so that you can pour footings.  You can frame out the area much like you would a deck.  This is given that the walls of the garage are above proper footings.  How many footings you need is dependent on how big your garage is.  How deep the footings are, is dependent of your climate.  

Also I would try to match up the floor to the floor in your house.  But all inspectors I know want ceilings to be at least 7 feet.  How close you can match these areas up is depended on your garage ceiling height and what you local town lets you do.