If your body is ready for it I would start lifting seriously as soon as possible. My biggest regret is not lifting in high school. When I did lift I had no idea what I was doing. I will give you my workout plan that I do here in college to give you an idea on what I do. Maybe you could use it for your self as a reference point.
Dumbbell Bench Press 5 Sets x 5 Reps
Dumbbell Incline Press 5x5
Incline Fly's 3x3
Barbell Bench Press 3x3
Pushups Until I Cant
Cable Tricep Pulldowns 5x10
Cable Tricep Extensions 5x10
Seated Tricep Dips 5x10
(After my workout I like to burnout with lighter weights if I'm not satisfied with my lift)
Barbell Deadlift 5x5
Chin Ups/Pulls Ups
Barbell Rows 5x5
Cable Rows 5x5
Lat Pulldowns 5x5
Machine Curls 3x10
E-z Bar Curls 5x8
Hammer Curls 3x10
Variation of other curls
Sometimes I will work the forearms with a barbell. (Burnout) Depending What time of year it is I will begin to power clean on back day as well. Have been slacking this year opposed to last.
Wednesday Legs/Shoulders (Can only do what my gym offers me)
Back Squat 5x5
Barbell Calf raises 3xburnout
Body weight calf raises 3xburnout
Box Steps Ups 5x8
Dumbbell Lunges 3x10 Count steps
Jump Rope 3x50 Hops
Shoulders I do lots of band work
Shoulder Press 3x5
Front Raise with weight plate 3xBurnout
Variation of other shoulder workouts
I prefer to get my cardio in after the I lift. I have been biking 10 to 12 minutes burning at least 100 calories. Abs after that every workout if I stick to it.
I may have forgot some workouts, but this will give you a foundation for lifting in general. This may not be a baseball workout specifically, but it will help you. I went from weighing 153 doing 30 pound dumbbells to 90 pound dumbbells in less than two years of lifting not counting my injury days where i couldnt do anything.
Hope this helps.