You’re 13, you are going to gain velocity annually just based on growth spurts. There is a good chance that you are going to experience joint pain (elbows, knees, hips etc…) as you grow, therefore I would caution getting to heavy into any lifting at 13. Pitching is an explosive repetition, so you may want to do some short sprints a few times a week. More than anything I like long toss workouts, there are some good ones, Alan Jaegers comes to mind. You are 13 so, listen to your arm. Any discomfort, back down. Start slowly. Good luck.