It is important to take the time to socialize young chicks so they become good citizens. Having animals that are well-socialized means that if you have visitors, it will be safe for them to move about your yard and property without worry that a chicken might come after them or cause a ruckus. Socialization is also useful when it comes to providing care; if a socialized chicken were to become injured, that bird would be much easier for you to collect and treat than a bird that is not socialized and has a fear of humans or aggression towards them.
Socializing is best started early in a chick's life. When they first hatch or arrive by mail, young chicks should be given some time to acclimate to their surroundings. There is a lot to take in, so give them a couple of days to explore and get comfortable until they are a few days old. At that point, begin sticking a clean hand into their box so they can get used to your presence. Initially, all you have to do is let them see your hand, although you may find that some chicks will waste no time and begin jumping on your hand pretty early into the introductory process. When you do move your hand about their environment, do so slowly so as not to startle them.
In a few more days' time as they begin to accept your presence, you can start offering food from your hand and allowing them to eat from it. While they do this, speak quietly and calmly. If you have a cue you wish to eventually have them respond to, such as a call to come for feeding times or to go into the coop at night, start saying it to them now. If they hear this word as they eat from your hand, they will likely associate not only that word but your voice with good things like food. As they get used to eating from your hand, begin to gently stroke them and attempt to pick them up. You do not have to remove them from the brooder, but simply cupping them in your hand a couple inches above ground level is a good start.
In a short amount of time, your chicks should start to greet you excitedly, mainly because they will anticipate food but also because a bond between them and you is being created. The shortest route to a chicken's heart is through the stomach after all, or so it sometimes seems! You can enhance their association with you that much more by offering the occasional treat, which they can also eat from your hand. Just be sure to offer grit once you introduce treats as it is necessary for digestion.
As time passes, continue to touch and talk to your chicks as they grow, picking them up and handling them so they will be accustomed to such behaviors and tolerate them without much fuss. The goal is for you and your touch to become what is normal to them so they accept and do not fear you. Human contact from an early age will make for chickens that are accepting and tolerant of people, which is useful in more ways than one. Visiting friends and family will not strike fear in your chickens and providing your own care or having a vet handle your chicken will be much easier with a socialized bird than one who is wary of your intentions, even those intentions that are good.