Aside from the obvious that people have been saying that I believe are true:
-all about calories in vs calories out.
-those extra calories retained per day should come from protien, so you want to make sure you are getting enough protien in your diet so your muscles have adequate building blocks.
-more testosterone = = more protien absorbsion, so increasing it builds more muscle.
How old are you? If you are out of your hormone raging years of your teens, Tribulus, Avena Sativa, and a Zinc supplement can increase free natural testosterone in your system. I wouldn't reccomend using any hyped up special formula gimmick supplement b/c as others have said, they are overpriced. Go over to
www.DPSNutrition.com and type in the names of the 3 supplements I just listed and choose the one based on best price to content ratio. What I do is count the # of servings and amount of key ingredient in each serving and weigh it against the price of the supplement. I like Ultimate Nutrition's Bulgarian Tribulus (750mg/pill @ 90 pills per container can't be beat). If DPS doesn't have the Avena Sativa you can google the name to find a high potency formula (high concentration of Avena per pill). Vitamin world also has Avena Sativa complex pills and Zinc pills if you don't want to go online.
How do these work? Tribulus works by increasing LH which in turn increases testosterone in males. There has been many studies on it and it has also been used for ages in natural medication. Avena Sativa is also known to raise LH, but not as much as Trib, but what it does a good job of doing is decreasing the amount of "bound" testosterone in your system that is bound to other compounds thus not available for muscle building. By increasing the amount of testosterone available for muscle building, you are essentialy giving yourself a boost. As you can see Tribulus and Avena work together.
The reasoning behind the Zinc supplementation is that people who are very physical and workout can tend to have deficiencies in Zinc b/c it gets depleted. Zinc supports proper testosterone levels, but it is no magic trigger for your body to create more.
None of those supplements I listed will shut your system down like steroids can potentially do. In fact, many people include these supplements in their post cycle supplement stack to get their grapefruits (in some cases, grapes) back up and running again.
The potential sides: any side effect you would get from increased testosterone could be witnessed...acne is a potential or possible raised estrogen, but I haven't experienced it. You don't want to take more than one stack of these at a time to avoid letting your body developing a tolerance/resistence to it. Also, the biggest thing you will notice is that you will have sex drive of a young teenager...don't let the hardons scare you too much

it is a good sign that it is working

.