fastest way to build muscle
The fastest way to build muscle is to create muscle confusion during your workouts.  Muscle confusion is where your muscles never get used to any of the exercises that you are doing.  This is important because if you do not create muscle confusion your muscles will get used to the exercises you are doing and will find the easiest way to perform them.  When this happens you will stop gaining muscle and you will reach what is known as a "plateau".  
Muscle confusion is one of the best ways to get bigger muscles, and to keep muscle "confusion" you will need to rotate your workouts every 5-6 weeks.  By doing this your muscles will never be able to fully adapt to workouts, thereby forcing them to work harder, which creates microscopic tears in your muscles during workouts, which your body then repairs and that is how muscles in your body grow.
Once you are done with your workouts you will want to ingest protein.  Your muscles are made of protein, and protein aids in repairing the small tears you have put in your muscles during your workout.  If you do not have enough protein your body will pull protein away from your other muscles to repair the muscles you have just worked out.  This is not what you want to happen, one muscle group to get bigger at the expense of another.  To avoid this from happening have a protein shake, protein bar, or lean protein source such as beef jerky after you workout.  Having an ample amount of protein in your body as it repairs itself from a workout is extremely important.
Another key to getting bigger muscles is making sure to not overwork your muscles.   When you workout a muscle group you need to give those muscles 48 hours in between workouts to repair themselves and grow.  If you workout the same muscles in your body less than 48 hours apart they will not have time to repair themselves properly and grow.  Make sure to give your muscles adequate rest in between workouts or you will only be doing more harm than good.
An important aspect to getting bigger muscles is to workout your legs even if you are aiming for a larger more muscular upper body.  The reason for this is that the more muscle groups you exercise at one time the more endorphins your body will release and the faster your muscles will grow.  Many guys especially do not realize that the fastest way to build muscle starts in your legs(which have the largest group of muscles in your body) because your body will release more endorphins to help your biceps, triceps, and pecs grow more quickly.  
Cardio exercises can also hinder your attempt at getting bigger muscles.  Cardio exercises can end up using your muscles as fuel if you're working too hard.  If you want a more lean, ripped look then you can up your cardio, but if you are going for bulk and want to put on size then you should limit your cardio exercises to 15 minutes 3 times a week.  If this is not hindering your muscle growth then you can up your cardio workouts in small increments.  Conversely if you are finding that you are having a tougher time putting on muscle then scale back your cardio workouts a little bit at a time where you can find that sweet spot of getting in the exercise you desire, but not setting back your muscle building workouts at the same time.
These tips on ways to get bigger muscles are just as useful for women as they are for men.  
Many men are more focused on putting muscle on their body than women, but women can put on lean muscle and get that toned look as well without having to worry about putting on too much bulk(men and women's bodies react differently to resistance training).  There are many ways to get bigger muscles, and this article outlines several methods that can help you reach your fitness goals.
-This article is not offering any medical/health advice.  Always consult your doctor before beginning any workout/exercise routine.
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