I'm going to assume you are saying your ceiling is drywalled/sheetrocked like the cieling in your house. This isn't as hard as it seems. The first thing is decide where you want to hang the bag.
Get a length of 2x6 or 2x4 thats around 4 feet long.
Ok whats next can be hard depending on what you know about finding studs(no jokes please)
What your are going to do is using a stud finder if your don't have one skip to below.
No stud finder. Mark where you want to put the bag on the ceiling, then take a small drill bit and drill in a straight line a series of holes no more than 1" apart til you hit something solid behind the drywall. This will be a rafter. Then take a tape measure and measure 16" from that point and do it again. If you don't find anything solid measure 24" from the first solid hole and do it agin. You should hit somthing. If you have access to the area above the ceiling another way to to go up there and drill down to the right side of 2 rafters then go back to the garage and use the holes as your starting point.
Find a rafter through the drywall near where you want to hand the bag. Then find one on each side. Take the 2by and span the distance screwing the board into all three rafters.(Just screw right through the ceiling and use a long enough screw to really dig into the rafter above. I would say 3 inches isn't too short. Also use at least 2 screws for each rafter (3 or 4 is better if using a 2x6)
Then take a eye hook, with a lag end, looks like a screw with a circle on one end. You want one at least 4 " long.
Drill a starter hole just shy of thelength of the eye hookfor it and screw it up through the 2 by and if possible into the center rafter(you should be able to tell where the center reafter is becuase of the screws. If you want it between the rafters thast fine to the spanner board will carry the weight.
