First you need to insulate it. Ceilings need R34 on top of 5/8th inch drywall. Walls are usually made with 4 inch studs so you do R13 over 5/8th inch drywall. You need to seal all the cracks so that takes drywall mud, figure a couple of gallons at least, and dry wall tape. If the doors are not insulated you need a door insulation kit for the doors. If you have a window and it is single pane glass you might want to put on an inner film barrier about an inch inside the glass. Check the gaskets around the doors to make sure they are tight. With all this, for a standard two car garage, if you do it yourself, you spent maybe $2000. Keep in mind that now it is pretty much air tight so you will need to use electric heaters. If you want to use a gas heater then you need to install a proper chimney or vent for it, that will run around $200. The best way to heat is gas using a double lined vent with an outside air intake so you are not using the warm air inside the garage to burn and throw out the vent. If you wall off half then you need to put in a standard studded wall with drywall and insulation on that. You will save a bunch on insulation but will spend it all on the new wall. I would not do that, stick with doing the whole garage. Also keep in mind if you live in the city you probably have to do a building permit. As a last thought, before you go and insulate and drywall the walls, add a bunch more electrical outlets so you have one about every 6-8 feet in the entire garage. Easier to do it before you put up the dry wall.