Strength training for sports is something that can start much earlier than 8th grade. We're not talking about Powerlifting, Body Building, or competative Weight Lifting but strength training. As for one posters take on heavy weights, heavy weights shouldn't be attempted until you are strong enough to lift them with good form. This comes from experience, not from age and applies if you're 10 years old or 80.
So, when can you start strength training? I like the Mayo Clinic's take on it http://www.mayoclinic.org/stre...raining/ART-20047758 The main take away is you can start early, emphasize form over weight, lift lighter but more reps (i.e. 1-3 sets of 10-12 reps @ 65%-80% 1RM), vary the exercises, and make sure you warm up, and stretch on the cool down. For those of you familiar with periodization (i.e. a condition phase, strength phase, and power phase) younger kids should concentrate on being in the condition phase, save the power phase work for when the growth plates have closed up (i.e. 15-17 yr).
For many sports strength training is combine with agility, quickness, and speed training. So we focus on movements that help build explosivness. This is true for baseball, football, basketball, volleyball, and holds for young men an women. The differences between sports are typically in what proportion of each exercise you do. So football linemen may do squats more often than a baseball center fielder and the center fielder may spend more time on doing falling starts (speed training routine) that the linemen, but both of them will do each exercise in their training program. So, where to start? First, you need to develop base strength and any good strength (or SAQ) program will get you there. You need to learn the basic lifts and exercises, how to warm up (a good dynamic warmup routine), nutrition (you need carbs for this, just eat the right ones in the appropriate amounts), and be monitored so that you're doing them correctly and safely. Start with weights (and reps) that you can do with good form. Eventually, you will be strong enough to engage in more challanging workouts, some that are baseball specific. A good rule of thumb strength measure is bench pressing your body weight (%85-%100) and squating twice your body weight (%150-%200). At that point you're strong enough to safely engage in more explosive exercises (a bunch of cool plyometric routines).
As for diet, if you're starting a strength program you might want to concentrate on eating correctly from now on and not trying to loose weight (which tends to reduce muscle). Concentrate more on inches of fat lost and a change in physique instead of weight reduction.
Important for any conidtioning program is warming up. I really like the dyamic warm-up routines from Nike SPARQ. They had training cards available for many sports and they were quite good (I have football, basketball, baseball, and general training). Any good dynamic warm-up routine is good, prior to lifting or other training. Windmills, high knees, but kicks, carioca, frankenstein walk, etc. are examples of dynamic warm-up exercises.
The following are a few resources you might find helpful
Books:
Weight Training For Dummies
Strength Training Anatomy
Training for Speed, Agility, and Quickness
Body Weight training:
http://www.trxtraining.com/
As for baseball the following clip is from a training program my son participates in. It's the strength training portion of a weekly pitching workout put out by Next Level Baseball MN, a training group comprised of coaches from a local D3 school. You'll get an idea of the use of free weights, body weight, and medicine balls in the context of developing pitchers http://www.youtube.com/watch?v=dp6n35d9UMs
Another resource I find interesting are the two workouts posted by Xan Barksdale on his Catching 101 site. I find the Atlanta Braves workout to be very informative and worth studing. I also like how it's for corner infielders, corner outfielders, and catchers. Very good stuff. It's not all appropriate for an 8th grader as it involves periodization but it is what you're trying to work up to.
Ole Miss http://www.catching-101.com/fr...ss-2003-workoutt.pdf
Atlanta Braves http://www.catching-101.com/fr...f-season-workout.pdf