The swing primarily uses the abs, back, legs, and forearms. This should give you some idea of what to train. There is also a study showing a correlation between bench press and bat speed. My son got into lifting using Starting Strength by Mark Rippetoe. He wrote a book, but there are plenty of free resources on youtube and bodybuilding.com among other sources that have all the information you need. It's basically 3x5 and upping the weight every workout by 5 lbs for bench 10 for squat and dead lift until you can't make 3x5. Lift 3 days a week and drink a lot of milk. Can't go wrong with 20% over 20% under for swing training. This can be accomplished by taping a sock filled with 100 pennies to a bat around the weight of your game bat. This should make it approx. 8oz heavier. You can use a fungo or an old -10 for your underweight bat. Swing the heavy 5 times immediately followed by the fungo 5 times. Swing with aggression. Take a minute break and repeat for 3 sets. Hand strengtheners are also a great way to build forearm strength. 2019 went from an 87ev at PBR to a 93 in one year doing this program.