new grower

so a friend just started growing recently and it's like a week in but i was just wonderin how much light a plant should get in the beginning? he's currently got it on 12/12

also, there was a problem with one of the seeds and two were planted in the same pot but one died, the only problem is that it's off centre in the pot. i'll grab a picture soon

Take them out of 12/12 and put them in 24 hour light or 18/6. You have to let them Veg for altleast a month, longer if you want tall plants. Leave the other plant alone, doesn't matter if it's to one side.

yes, use 24/0 or 18/6

what about CO2, I heard something about using CO2 for a higher yield. how's that done?

You can make your own CO2 by putting equal parts of water and sugar, and one packet yeast into a container... Poke holes into the top of the lid before you put the top on and mix though. Hang this up above your plants (CO2 is heavier than air and will fall when released from the container) while the lights are on (wont absorb CO2 while the lights are off), and give it a shake or add a little water to refresh.

Should last you around a 4-5 days. There are also more high tech ways to go... Buy a CO2 tank and regulator, but this would be for a more seasoned grower...

when's the best time to use this? or is it something that the plant would benefit from anytime?

In my experience CO2 is best used during vegetative growth, and as mentioned above, when the lights are on. Plants use CO2 and kick out O2 though, so the full duration of your grow would benefit from the use of CO2.