There are several ways to generate CO2 for your aquarium:
The most straightforward method is to use a compressed gas bottle, regulator and a fine "needle valve". This method cost a bit more than the others, but it is undoubtedly the most reliable one.
DIY Yeast Source: A very cheap and commonly used source for CO2 is simply mixing sugar and yeast in a bottle of water (3/4 cup sugar, 6 cups water and a pinch of yeast - roughly 1 teaspoon). Although cheap, this method has some disadvantages: the mixture has to be replaced every ~10 days, and there is a danger of yeast mixture reaching your aquarium water.
Though the gas cylinder and the yeast bottle are the most popular sources, there are products coming out on the market every year using alternate means of producing the gas. These products usually relay on some chemical reaction to produce CO2 like the Tetra CO2 Depot.
Another, somewhat strange method, is to use human respiration. Although, your breath has a low concentration of CO2 (about 2%) you can provide some of the gas with your own breath - read more.
