The most important aspect of gaining lean muscle is your diet. Straight up. You can't gain lean muscle without eating a healthy, high-protein diet. Your body needs you to intake plenty of protein so that it doesn't need to use your muscle for energy, and the amino acids in the protein will build your muscles up stronger assuming that you have an intense workout that breaks down your muscles.
There are three types of protein supplements common to the market: Whey, Casein, and Soy.
1. Whey protein generally has a greater number of amino acids neccessary for muscle recovery.
2. Casein protein is a slow digesting protein, perfect to take at night so that your muscles recover efficiently while you sleep.
3. Soy protein is, obviously, from soybeans and honestly, I've never taken any Soy protein supplements. I do, however, eat soybeans (soynuts) as they are high in protein and fiber.
So now on to the big question: which protein powder is the best? Muscle milk, Optimum Nutrition, Syntha-6, GNC, Body Fortress, etc, etc? This can vary significantly depending on your goals, I personally prefer Syntha-6 because it contains some complex carbs, and some fat to increase your calorie intake for the day. Eating healthy foods sometimes is difficult, and it's nice to have a protein shake that can knock out 400+ calories.
I reccommend taking 1 gram of protein per pound of fat. More than that seems goofy to me, because more than that jacks your calorie intake up too much. Also, on a side-note wikipedia says your body can only digest 120g to 220g per day...nuff said.
Also, don't mix more than one scoop of powder per shake. Per serving you should not have more than 30 grams of protein, most research suggests your body can only digest around 20-30 grams of protein at a time.
That's that. Peace.