Ideally the best method of cleaning is to remove the door and clean out the track using soap and water. You can do this in place as well, but its harder to remove the grit and dirt that collects in the track.
The door rolls on wheels. These wheels are adjustable from each end. Normally you will find a little button on the inside bottom about three inches from each end. When you remove this button you will discover a small screw. Turning this screw will raise or lower the wheels thus adjust the door up or down. Some older doors have these screws on the edge also.
In any case if you have some help, you should be able to turn the screws until the rollers have lowered the door as far as it will go. Once this happens the door at the top should have cleared by just a tiny bit the top of the frame. Tilt the door in carefully and you can remove it from the frame. Set the door carefully on its edge so you have access to the rollers on bottom. You may need to adjust them once again so they come out of the door itself. Then you can clean the rollers, and lubricate them using some light spray oil so the wheels roll easy. in some cases you will find the rollers being flat on one edge. This happens when they stick and drag on the door frame track. If this occurs, find out the manufacture of the door, which is normally found on the door handle and contact a local supplier of this door and purchase new rollers. They normally come out easy and can be replaced easy. If you must wait on getting them simply retract the rollers back into the door and set it back into the frame.
Once the door is reset into the frame, you turn each screw approx the same amount of turns as the door moves back up into the upper track. then open and close the door as you continue to adjust the door until it closes even with the frame and seals properly with the fixed door unit.
What ever you do, do not lubricate the track. People make this mistake all the time. The result is dust collection that becomes hard to remove and builds up fast. The only item that should be lubricated is the door lock and rollers.