Walls
Many garages have open studs or finished walls with little insulation in them.
Ceiling / attic
Heat comes down from the roof. Ceiling insulation slows it.
Insulated garage door
A door with an insulated core, or an insulation kit for your current door.
Seals
Bottom seals and side weatherstripping.
Related: garage mini-splits
FAQ
Should I insulate before adding AC?
It's usually worth discussing together. Insulation lowers the heat load, which can affect the size of unit you need.
What about the garage door itself?
An uninsulated metal door can be one of the biggest heat sources, especially if it faces west. Options include replacing it with an insulated door or adding an insulation kit. Ask a garage door pro what fits yours.
Does weatherstripping matter?
Yes. Gaps around the door and at the bottom seal let hot air and dust in. Seals are a small but useful fix.