You dont necessarily need it, but you might want it if you have a tank with high light levels. I run planted tanks without it and have done for a few years.
There are different ways of doing it, ranging from simple fermentation of yeast delivered from a bottle to pressured cylinders of gas. Its hard to do justice to it in one post. Google it and read around the subject a bit.
In the meantime, what is your set up (tank size lights etc) and what plants are you growing?
Okay will do. My tank is a 450L with 2x 42" 40W Aqua Glo Lights and the following plants Java Fern x2 Good handfull of Java Moss Moss Ball x 2 Amazonian Sword Plant 2 Plants that are growing on the bogwood And LOTS of bogwood I dont have a clue what plant is what at the moment! but... Nomaphila Stricta Thai Anubias CongensisHygrophila Pinnatifida Rotala Wallichii Ophiopogon Kyoto Hydrocotyle Leucocephalis
I can probably save you some time having read the tank specs. Your tank is too big for yeast based CO2 injection or any of the sub ?30 CO2 starter kits to be effective so you can forget them. However your lighting levels are too low to justify a pressurised CO2 system. You can dose with liquid carbon if you like but if you want to change anything you'll probably get the most benefit from uprating the lights. On the other hand, if things are working right now and you're not hankering to spend money and try something new - leave it alone and be happy :)