What's the best way to fill in some uneven cracks in my garage and level it out so that I can paint over it?
The paint may cause you trouble, as most want concrete to have cured for 30 days, so make sure that the brand you use will work with your timetable.

The concrete repair products are best for cracks up to 25mm and similar depth. Larger than that would require a concrete patch 1st, filled to within 25mm of the surface. Then the repair patch (just a sanded mortar type) or a refinisher patch product, which can be troweled paper thin.

A concrete bonding agent (usually a latex based product) is used first, especially if any sealing was done previously.