Just go for overall strength training without getting too complicated at the moment. Learn movements that will help, and make sure that shoulder stability of the shoulder joint, especially rotator cuff are strengthened. Softball isnt as bad on the shoulder as baseball, but its getting there.
Learn Big multijoint movements, but dont go too heavy, at 13 she is a late developer if she is pre-pubescent, weight training around puberty will be fine, and will be excellent for bone mineral density.
Dont go too complicated to start with (like OLs) as they take a long time to learn and strength can be gained in other simpler movements to start with.