First question I would get answered is what type body do you have Mesomorph,ectomorph etc...Some people are not capable of putting on more than a certain amount of size so lifting heavy will not do much except injure you. Talk to a trainer at the gym they can help you figure this out.
In the short run if you are looking for muscle strength for baseball I would do 8-10 reps at a medium weight for 3 days (break your routine into chest/tris, back/bis, legs and shoulders), taking one day off and then doing 3 days of light wieghts 12-15 reps. I play a lot of hockey and this gives me a good strength/endurance mix. I would also suggest at least 20 minutes of cardio (running or elliptical) a day to get your body in shape...and avoid slurpies
Hope this helps.