iv got the zoo meds 17 watt max plant growth light and i use a iquid by Seachem called Florish Comprehensive supplement for the planted aquariumhow much light do you have?. thats a major factor in weather or not you need co2 supplements
(i dont have a co2 system, but iv read up about them- and im hopeing to get one in the future, if my parents let me)
some plants need more co2 than others.
there are 3 main ways of adding co2 to the aquarium
pressurised
biological
and liquid