What kind of formula should i give my one month baby?

when my baby was born i was giving him newborn enfamil but its 0-3 months i want one that's for the 12 months so i don't have to be changing it all the time and right now im giving him the gent lease for fussiness and gas but it seems like he dint like it also ever since i gave him that he poop green so i don't know what to do should i go back to newborn and then chang it also i heard that changing the formulas not good can cause stomach pains help?