best way to get arm strength personally is do a few things:
#1: Determine your diet
-The biggest problem nowadays is people feel that since they lifted weights or whatever for a hour they can stuff their faces with random shit for the next 2 days as long as they gain no weight, guess what WRONG.
-The key to building muscle mass is getting high protein and mid fat / carbs but dont completely cut them because you need all 3 to build muscle mass.
-If you want to go more in depth on diets etc go to this site its a nice guideline personally:
http://t-mag.com/html/147mass.html
#2: Weight lifting itself
-Another big problem people have is they focus on one area of their body to improve but dont improve others which can cause major problems. If you dont exercise your entire body some parts of you can become to bulky and you can become overall weaker because your legs cant support your arms as easily.
-For arm strength focus on exercising your back for flexibility and do alot of Curls/Bench presses however things like squats do not hurt since speed is key in baseball also.
Honestly if you combine these two you will have the best results however some things are decided entirely by your own bodies, some people react differently to different diets since you will be doing this for 6 months and longer for baseball give it time and see what gives the best results.
However, I advise you see a trainer/doctor just to get their opinion but in general most people will get stronger without gaining body fat doing these 2 things