This is a "piece of string" question impossible to answer !
It all depends on plants verses lighting.... you either choose plants to "fit" your light, or lights to "fit" the plants - its up to YOU which way to go !
You can get plants that require very little light, and may well survive with your current setup, but most aquarium plants originate in the tropics - same as the fish, where there is bright daylight... this can be simulated in several ways, "daylight" energy saving lamps or fluorescent tubes designed for aquarium use are the simplest method (and cheapest !)
Again the amount of light will depend on the plants you are choosing to some extent... some like a lot, some not.
Get a daylight lamp fitted (what fittings are there already ?) and choose some standard plants known to be hardy... you dont "need" CO2 AT ALL unless you have loads of light, and are prepared to add fertilizers etc.... look upon it as another hobby, its not that hard to achieve but requires a little more effort + cost.
If all you want is "some plants" then forget about it altogether.
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