trimming so i can get a few big buds rather than loads of small ones

this is my second grow now my first went ok but the yield was not brilliant but ok
just wondering what part of my plants i need to trim so i can get some nice buds?
also one of my last plants grew really tall where as the other was small with 4 big buds on it
how can i keep them smaller?
ive just bought this co2 machine will it help?

ok nobody?

the co2 will help, only use it when the lights are on. if you go to grow faq theres a bunch of info and pics on pruning.