Your dream car is parked in out of the elements, but it's not secure because the garage door won't lock. Most garage doors have two horizontal bars that move out from the center of the door into slots along the side of the door in the door track, effectively locking the door in place. Over time, these bars can shift slightly out of position so that they are no longer correctly aligned with the locking slots. To realign the bars, unscrew the guide brackets on the edges of the door so that they are loose enough to move, and then reposition them so that they smoothly guide the locking bars into the locking slots. Lubricate the lock mechanism with machine oil and you're done.
I'm not a professional, but I watched as my garage doors were serviced last year. The tech used a spray lubricant (I had bought a different brand in the garage door section of a home improvement store). He sprayed the inside of the entire track, top and bottom, the rollers that ride along the track, and all hinges on the door. He sprayed each hinge twice -- once on each side. The techs were more generous with the lubricant than I was when I'd done it before, so don't be afraid to give it a good squirt.
Garage Door Opener