While you should work out many muscles, you do not need to work out all of them at once.  In fact, stimulating the same muscles extensively two days in a row is actually bad for them and will hinder your improvement.  That is why people use different machines on different days, in order to give certain muscles a rest while working out others.  Remember, the one-day principle only applies to muscles and not the whole body, so you can still work out every day if you prefer, you just have to change up how you work out.

There are many muscles on the arm that can be strengthened.  There is the bicep, tricep, and shoulder, and if you go even further down the arm, you can count your hand and fingers.  Each muscle is stimulated when force is applied to it.  There are some workout machines that can work out a multitude of muscles at the same time, while others focus on one.  In the latter case, since the force is not divided, you get more benefit for that muscle, but it takes longer to improve everything you want.

Next is the middle of your body, which includes the chest and abs.  These muscles can be strengthened by using such devices as the chest press and a traditional bench.  For the abs specifically, you can simply do sit-ups or push-ups, and those exercises combined with the use of home gym equipment will result in a lot more muscle gain and improvement.  Keep in mind many of the exercises for the chest can work out some muscles in the back, so you get dual benefits from certain routines.

Finally, you have the lower body.  This includes all the muscles in the legs, including the thighs and hamstrings.  Exercises have been designed to work out every muscle in this group, and the leg specifically can have many muscles worked out at the same time.  By exercising in this fashion and being smart about your routines, you can improve your body very quickly and efficiently. By the way, if you want to be really successful, you should consider using a personal training business.