Add expansion joints on garage floor - too late?
You can cut expansion joints with something like this:



This is a walk-behind concrete saw that can be rented at most rental centers (I know that home depot has them).

After the joints are cut, fill them with a good polyurethane caulking.

I would wait until after you cut the joints before filling the existing cracks, as it's likely the work you're doing with the saw will result in vibration or "abuse" of the floor and might widen existing cracks.