The plight of the American male: how do I get big and gain muscle? I hear it everyday. While the answer to the question is different for everyone, a lot of the same principles can be used successfully for the vast majority of us.
This probably one of the most difficult to grasp while at the same time the most important. Everyone wants fast results, they want to workout today and wake up tomorrow looking like Arnold. The reality is: it just doesn't work that way. Skeletal muscle is one of the most metabolically expensive tissues in our body; it burns calories even when we're at rest. Our bodies operate as efficiently as possible and will always try and use the fewest resources possible. So if we want to gain muscle we need ever-increasing stimulus over a long period of time. Building muscle takes a life-long commitment to hard work and taking care of your body. That being said, a novice lifter is going to see results fairly quickly if he or she sticks with it, so don't be too discouraged.
2. Don't expect to lose fat simultaneously
The idea that our bodies can build muscle while burning fat is a result of disinformation distributed by the supplement industry. Our bodies can either be in a catabolic state (breaking down) or in an anabolic state (building up), but not both at once. That being said, if you want to pack on a decent amount of muscle you need to ensure you're eating enough calories.
3. Be Consistent
This goes along with #1. A lot of the time people won't see their desired results quickly enough and will change workout routines on an almost weekly basis. If you're workout plan is structured properly, you should only change things up at 12 week intervals. Don't change horses midstream, stick with it.
The squat is by far the most important lift to anyone that is trying to gain muscle. Squats work every muscle in the lower body and force the muscles of your upper body to stabilize a tremendous amount of weight. If you've ever heard the expression "squat for big biceps" it definitely has a ring of truth to it.
5. Drink Milk
Preferably whole milk. Milk and dairy products in general have a huge anabolic effect for a variety of reasons. When I'm training teenage athletes I recommend they drink as much as a gallon of whole milk a day, on training days.
6. Check your Ego
This may be more of a muscle gain killer than lack of patience. Don't go into the gym and try to lift as much as the guy next to you. Use weight that you can handle with proper form if you want to get the best results. Exercise form is extremely important not only for injury prevention, but also for muscle gain. If you aren't isolating the correct muscles with each lift your body will use other muscles to help out, diminishing the gains you will see from your efforts. We've all been beginners and no one is going to judge you if you aren't the strongest guy in the gym.
While, those are definitely the top 6 (it was going to be top 5, but I thought the ego thing was important) I've only begun to scratch the surface of muscle gain. I recommend you make an appointment with a qualified fitness professional to discuss your goals. Pick up a copy of Starting Strength, you can find it on amazon and it's a great place to begin your journey.