My son went through the same thing when he started to think more seriously about pitching and starting some kind of off season/pre season workout routine.
I would suggest starting with what you want to achieve. What do you want to gain from your workout, flexibility, strength, size, endurance… ? I’m assuming this is baseball specific, pitching focused?
I would suggest avoiding goals like gaining more velocity. Focus on goals specific to your training regime, achieve those goals, and then work on your mechanics to let your added strength, flexibility, etc… express themselves in your pitching.
The first thing to understand is that you’re not going to get a generic training regime recommendation from most people who are serious about working out on these boards.
Your training regime needs to be specific to you and your needs.
When my son started looking into a workout routine, he and I had a lot of the same fears and believed some of the same myths about working out with weights that are prevalent in the general community.
Working out with weights should not be a problem at your age, but working out with weights and lifting incorrectly or lifting too much weight without maintaining correct form or choosing the wrong kinds of weight lifting exercises for your needs would cause problems.
Look, do a little research on weight lifting exercises that baseball players/pitchers do to keep themselves in shape.
Kyleb and others turned my son and I on to Eric Cressey’s web site as a reference: http://ericcressey.com/baseball-content
He has some excellent content and an expert’s perspective on some do’s and don’ts in developing a workout routine.
My son’s routine is built around developing core and leg strength while improving flexibility and range of motion. His upper body routine is based around working with dumbbells; medicine ball exercises; and non weight lifting exercises like pushups, pullups, and chinups; and lots of stretching.
I would suggest finding a range of exercises, stretching, and cardio work that fits together in a program for you. Then make sure that you start slowly and learn how to do everything correctly and then do your exercises correctly.
A lot people spend a lot of wasted effort in doing exercises incorrectly, which can be a waste of time and effort and worse a cause of injury.