Once your child can begin understanding what you are saying, they are ready to learn manners! The standard "please" and "thank you" can be taught to a child who is still not yet speaking, but is using sign language instead. Politeness and thinking of others does not come naturally to anyone, especially young children! It is a skill that must be practiced over and over again to places and people that are meaningful to your child.
Young children need much practice with learning to wait their turn and not interrupt another adult who is speaking. Teach them to touch your arm if they have something they would like to tell you while you are talking with someone. Likewise, touching their hand in return can communicate to them that you acknowledge their presence and that they have something to say to you once you are finished talking.
Play dates and interacting with their friends is a great stage to practice manners and being polite. Have your child ask questions such as, "Would you like to come and play outside?" Teach children to come to the door and greet anyone who comes in. When a friend is leaving to go home, have your child be the one to tell them thank you for coming over and playing. Turn taking is another skill and manner that can be practiced during this time, and play dates provide ample opportunity for this.
Younger children also many need practice using eye contact when speaking to someone. Toddlers and young preschoolers can often time be distracted and not focused on the person speaking with them. Practice eye contact in the home, and help your child by telling them to "look at Grandma's eyes and tell her hello."
Older children can offer help in additional ways that younger children may not yet be able to. They can greet house guests at the door, take coats and purses to a designated area, and offer a beverage if appropriate. They can also assist guests when leaving by offering to carry items to the car, retrieve purses and coats, and anything else they might need. Older children must also be taught proper phone etiquette - what to say, how to say it, and who to say it to - and this is something that can be established by the parents. Grandparents, aunts and uncles can be great helpers in providing role play examples for kids to practice phone etiquette.
This list for practicing manners for young (and older) children might feel overwhelming and even burdensome at times. Focus on just a handful at a time, and realize that change doesn't happen overnight. Be patient with your child, praise them in their successes, and patiently teach them (rather than berate or punish) in their mistakes.