As you increase the light in your tank, and therefore the available energy, you'll find that eventually the plants will use almost all the available CO2 in the water, and in order to keep the plants at optimal health and growth (and to minimize algae), you'll have to inject CO2 into the water.
There are two general ways of doing this -- they both involve bubbling CO2 gas into the tank, but the gas generation method is different.
There's the DIY method, which uses a mixture of sugar, yeast, and water to produce CO2. Such a system is usually easy to assemble, dirt cheap to make and run, but it can get messy and inconvenient, especially for larger tanks. A quick web search for "DIY CO2" should find a few good sets of details.
Here is a good article about DIY CO2, to get you started.
The other way of generating the CO2 gas is to buy it, in pressurized bottles -- generally, from a welding supply store or a beverage distributor. When you use a pressurized bottle, though, you'll need some hardware -- namely, a regulator which will let the CO2 out of the bottle at a predetermined (fairly slow) rate (edit: and in most cases you'll need a needle valve, too, to finely tune the flow rate). Some regulators come with bubble counters (which let you see how much gas is actually coming out of the bottle) and/or solenoids to allow you to turn the CO2 flow off and on with an electric signal. Of course, if you want your flow to be controlled automatically, you need to either put it on a timer, or you have to have a sensor and controller (usually pH is used as the factor).
Here is Chuck's guide to pressurized CO2, which does a much better job of explaining it.
As you might guess, the cost of these little doodads start to add up pretty quickly. At the least, you'll probably shell out $150 to $200 to get started, and with automatic controls, it's fairly easy to top a $400 price tag.