The big thing is to determine what you want in a chicken. Good natured docile pets, high volume egg layers, good foraging skills, cold hardy, able to take the heat? Take your time and research breeds. No, not all chickens in a breed have the same personality. But kind it stacks the odds in your favor for typical breed characteristics. You know, like someone getting a golden retriever without doing any research and then being shocked when it won' stay out of the water. There are a lot of opinions, so be prepared to spend a good amount of time doing research and reading up. There are so many breeds and its fun to think about all the possibilities. Then get your layout (coop,pen, etc) set up first. Far too many of us (meaning, me) have bought the chicks then scrambled around desperately trying to get everything set up by the time the chicks are ready to go outside. It always takes longer to get set up than you think and if you recruit a husband or other man to help, it will take even longer.
There are a lot of threads on introducing new birds to the flock and lot of tricks for doing so. I started out this spring with 18 chicks of 10 different breeds (I had no idea what I really wanted), then sold off some pullets and a couple of accidental roosters at about 10 weeks old, keeping only the nice, easy-going girls. Sold off a couple more polish at about 5 months- just too flighty for my taste. I had 5 adult pullets left that I really,really liked (1 australorp, one speckled sussex, 1 welsummer, 1 silver laced wyandotte and one gold laced wyandotte). When the hens were about 5 months old, I added 15 silkie chicks I had hatched -fresh just out of the brooder- at about 6 weeks old. I have a good sized pen (15 x 35) with plenty of places for chicks to hide and I made some more hidey spots the big ones could not get into. I think the sheer number of chicks overwhelmed the adults. I had a little chasing and a minor peck or two but not much more than enforcing pecking order. No injuries at all. Not even missing feathers. They have been together nearly 2 months now and are slowly melding into one flock. The babies are a little over 3 months old now. They often hang out as two separate groups but when I bring treats, they all gather around my feet and eat together without too much fussing. I'm noticing them hanging together just a bit more and I think in time they will all be one flock. On the other hand I've heard a lot of horror stories about chickens killing a new one, so it can happen.