There are numerous exercise that you can workout in a gym. Each exercise is either for a specific muscle called an isolation exercise or compound exercise where multiple muscles are worked together. Which muscle should you train first or what combination of muscles should you take for more training. Training different muscles depends upon what your objective is and your approach to muscle training.
There are certain number of things one should keep in mind while training his/her muscles. When lifting weights it is most important and foremost to maintain the right posture and perform the exercise in the right manner. Doing them in the wrong posture could unbalance the symmetrical growth of the muscles and could cause injuries and pain. If you are looking for muscle mass then lift more heavy weights with which you could do only 7-8 reps of the exercise. For defining and better cuts of the muscles don't rush through your session. Change your exercise pattern at times as the body can adapt to the pattern. Rest is important as muscles require time to repair the wear and tear and increase in size and strength.
Work Bigger Muscles First: There are various combinations that you can take while building muscles. There are big muscles in the body like chest, back, thighs, abs and small muscles are biceps, forearms, shoulders, triceps etc. If you work on the small muscles first then working the bigger muscles will be straining as smaller muscles assist in training the bigger muscles. If the small muscles get tired then it will be less supportive for bigger muscles. So do the bigger muscles first and then move on to smaller one. This gives more muscle mass and density to the muscles with strength too.
Combine Opposing muscles: If you are looking to increase strength in your muscles to lift heavier weights then train opposing muscles to concentrate more on the muscles and provide them enough rest. Opposing muscles are least incorporated while doing the other muscle so its gives you time and strength to work the opposing muscles with heavier weight. This style allows you to work more muscles in a workout. Suppose you work your chest then the biceps don't play much role in it. So it is possible to work the biceps as well with full power and weights.
Work the supporting muscles: Work out combining muscle group in one workout. The muscles that support each other. Like biceps and back. Chest and triceps etc. This helps build muscle endurance, gives better shape to the muscles. Training supporting muscles works the bigger and small muscles as well. It gives a right combination to exhaust the muscles for better and ripped muscle mass.