Rest is a very critical part of bodybuilding- at any level. Actually, rest is where the actual "getting bigger" takes place. So it is very important to organize your program so that each muscle is trained for the maximum amount of growth possible. Some muscles combine better with others in training. Also, it is important to make sure that your workouts don't conflict with each other. For example, if you do lower back the day before you do legs, you will have a lot of trouble doing squats because your lower back is needed. So if you don't organize it all correctly you will have to take a lot of extra days off (especially in the example I mentioned because the lower back takes longer than any other muscle to recover).
I must note that I am not a strong believer in overtraining, or at least what some people consider overtraining. I believe that overtraining most often results from improper nutrition/supplementation and/or from training a muscle too frequently. Some people are set on the idea that 12 sets or more for any major muscle group is far too many, and that you can completely stimulate all the available muscle fibers with one (or rarely a few) long, concentrated sets of each exercise. Now I agree with this to an extent, as I don't believe that you should do 15 or 20 sets when you first get started, but 1 or 2 sets of bench?! of squats?!. Now I know how tired I am after a set where I take it to absolute failure; but the thing is I can always rest and do a few more- meaning that not all the fibers were exhausted. I simply believe that performing 3-5 sets for each exercise will recruit and overload the most fibers, thus, producing the greatest gains.
Now- to the training split. Your muscles really don't know what day it is so I don't have specific days for each muscle. I also don't believe in scheduling rest days- I take them as they come. I just hate it when you schedule a rest day on Monday and plan on resuming on Tuesday, and come Tuesday something comes up and you can't work out. Now if you were sore and needed a break on Monday then oh well, it was necessary; but if you could have worked out on Monday and didn't simply because it was on the schedule, well then you should kick yourself.
Ok finally- here's the split...
Day 1: Quads
The only muscle that really shouldn't be sore when training legs is the lower-back because you need this very much when doing squats. Some people prefer to train lower-back with legs, but you need your lower-back in order to train your upper-back. So rather than training them all in one day or taking off days to let your lower-back recover, I believe training it with back would be better.
I am a firm believer that the quadriceps are by far the most demanding muscle group in the body to train. Because of this I devote an entire workout to them. The great thing about legs is that there is no question as to which exercise will give you tremendous gains. The squat is the absolute best exercise for developing your quads and is also one of the fundamental compound exercises in your routine. However, this is a subject that I could talk about for hours and before I get too far I'll continue the with the training split (if I am accepted to write for Bodybuilding.com, I'll have an article devoted to each major muscle group within a few months).
Day 2: Shoulders and Traps
Your shoulders should be fresh when you train either your chest or triceps. You need your front delts when performing virtually all chest exercises and for some triceps exercises. You really don't need any help from your chest to train shoulders, but you do need your triceps to perform many of the shoulder exercises. So it is a good idea to do your shoulders right after your chest or long enough before it to have fully recovered. As for triceps, hit your delts right before them or long enough after them to have recovered. The traps are involved in virtually all upper-body movements that require you too hold a weight while standing. Your traps really aren't needed heavily in much of anything except for shoulders (where they are required, and that is why they are trained together). However, they are needed in some exercises such as deadlifts and act as stabilizers, so be aware of this when going especially heavy.
Day 3: Back (both lower and upper), Hamstrings, and Biceps
You need your biceps and lower back to train upper back- so this is the exact reason we're training them together here. Your lower back is very unique and is unlike any other major muscle group in the body. It's sole purpose is to act as a stabilizer, holding the body steady rather than contracting through a full range of motion. Therefore, when you do work it through a full range of motion with good mornings or hyperextensions, it can become so overwhelmed that it can take a week to fully recover. Hamstrings tie right in with the lower and upper back. With this workout, you do your back first, which uses the lower back as a stabilizer. Then you immediately hit your lower back using stiff-legged deadlifts as the power movement, which also involves the hamstrings so you can go ahead and do 3-5 sets of leg curls. Next, just do a few sets of barbell curls and then an isolation exercise to finish of your biceps which are already a bit pumped from hitting your back.. When this is over, you should feel very satisfied; you just eliminated 4 muscles from the split.
Day 4: Chest and Triceps
After hitting your shoulders a couple days ago, they should be ready to assist in this workout. The pectorals and triceps work very well together- even better than the biceps do with the back. The chest is a definite favorite among teens, while the triceps are one of the most neglected. I hate to see this because these are my two favorite muscle groups. The chest is just my all-time favorite, but the triceps are my favorite to train. Just like with the back, you are blasting the chest, and then finishing off the already tired triceps.
*Abs and Calves
When you train these is totally up to your preference. For calves and abs, you really don't need any secondary assistance from any other major muscles group, so train them whenever they're ready. I personally believe in training your abs whenever they're not sore. Since they only take about 10-20 minutes to train, just hit them right before another workout or as often as they are ready. Now calves can be done on the chest day, shoulder day, or on the leg day (the back day is already pushing it). Since I like to focus on only my legs when I hit them, and that shoulder workout usually kills me by the end, I usually do my calves at the beginning or end of the chest and tris workout. Another thing I sometimes do is if I'm about to take a shower anyway, I'll just load the bar up and do my calves real quick separate from the main workout on any one of the days (this is only because I pump at home though).
This split is designed so that someone could pretty much work out everyday, but I am definitely not suggesting this. I am just merely organizing a plan so that you can workout as often as your body allows you to at your particular level. Someone just starting out should not lift more than 5 days per week- but this is even pushing it. You must gradually prepare the body for the changes that you are planning on subjecting it to. However, you must also remember that the body is amazingly adaptable, so pushing yourself to the limit is not a bad thing. Just make sure that you do your research. You must learn as much as possible through reading and experimentation. This site alone has enough information to start you on your way, so you can be the person giving the advise. One important thing to remember is that everyone has their own opinion, and it is best to examine as many as possible. Good luck and keep pumpin'.
"The pain of bodybuilding is inevitable, but whether you suffer or not is entirely up to you."