best cost/performance ratio will be changing to a stronger filter. depending on the size of your tank, i would suggest an emperor. you can go canister which is even better but i love my HOB
dont need co2
unless you are doing a heavily planted tank.
new lighting does make a difference. being new to the hobby, i would do a lot of research into what sort of lighting you would need. do you want a VHO
? compact? lighting can be expensive. a better cost/performance upgrade would be to just change the stock bulb to one that outputs more or gives off a different light color.
as for new lighting, you can do the simple/effective bulb change or you can get a totally new bulb enclosure.
whatever you do, do your research and dont impulse buy.