It really depends on the market in your surrounding area.
Is it best to sell eggs, birds for meat, or eventually buy an incubator to sell chicks for new farmers?
Ultimately it is most profitable to eventually sell chicks if you have the market, but I would start by selling chickens for meat at the age of maturity 4-5.5 months depending on the breed. Makes sure you feed them properly, know the basics of running the farm beforehand, and spend the time to find a buyer in your area before the birds are mature.
eLengo also offers an online course in poultry farming if you are interested!
Find out more information here.