! Great to have you aboard! To intro new chickens to an existing flock, give the two groups access but not actual contact - like through a fence - for a few days. After this put the new ones into the coop at dark with the old ones - they won't pick on each other in the dark. After spending the night in the coop open the door at first light so no one is trapped in the coop! Let them mingle - there will be pecking order squabbles - that is to be expected. Should work out fine! Good luck to you!
Good suggestions above - don't be alarmed if the younger chickens get bullied, it's pretty normal. They'll learn how to stand up for themselves soon enough. I find it's best to feed the two groups separately, because our bigger chickens will just spread out and eat all the little chickens' food. So we feed the little ones outside the coop area.