What muscles should I work out to swing a heavier bat?

Im 14 in 8th grade and i play baseball for my little league. When i move up to high school im gonna play baseball, but they require that you use a -3 bat (a drop 3 bat). A drop 3 bat is too heavy for me, so i've realized that i need to hit the weight room. What muscles should i work out to swing a heavier bat? Thank you.|||Pectoral, deltoid, biceps and teres major muscles.