Here is a very good guide to what you should know about politeness and manners. Following these guidelines is not only necessary it is required to be thought of as cultured and civilized.
If you don't want to read through them all here they are listed here: http://shine.yahoo.com/channel/parenting/25-manners-every-kid-should-know-by-age-9-2480238/
When asking for something, say "Please."
When receiving something, say "Thank you."
Related: Kid-Made Thank You Notes
Do not interrupt grown-ups who are speaking with each other unless there is an emergency. They will notice you and respond when they are finished talking.
If you do need to get somebody's attention right away, the phrase "excuse me" is the most polite way for you to enter the conversation.
When you have any doubt about doing something, ask permission first. It can save you from many hours of grief later.
The world is not interested in what you dislike. Keep negative opinions to yourself, or between you and your friends, and out of earshot of adults.
Do not comment on other people's physical characteristics unless, of course, it's to compliment them, which is always welcome.
Related: Raise Polite Kids
When people ask you how you are, tell them and then ask them how they are.
When you have spent time at your friend's house, remember to thank his or her parents for having you over and for the good time you had.
Knock on closed doors -- and wait to see if there's a response -- before entering.
When you make a phone call, introduce yourself first and then ask if you can speak with the person you are calling.
Be appreciative and say "thank you" for any gift you receive. In the age of e-mail, a handwritten thank-you note can have a powerful effect.
Related: Print and Color Cards for Birthdays, Thank-Yous and More!
Never use foul language in front of adults. Grown-ups already know all those words, and they find them boring and unpleasant.
Don't call people mean names.
Do not make fun of anyone for any reason. Teasing shows others you are weak, and ganging up on someone else is cruel.
Related: Raise a Compassionate Kid
Even if a play or an assembly is boring, sit through it quietly and pretend that you are interested. The performers and presenters are doing their best.
If you bump into somebody, immediately say "Excuse me."
Related: Quiz: What's Your Parenting Style?
Cover your mouth when you cough or sneeze, and don't pick your nose in public.
Related: How to Handle Inappropriate Behavior
As you walk through a door, look to see if you can hold it open for someone else.
If you come across a parent, a teacher, or a neighbor working on something, ask if you can help. If they say "yes," do so -- you may learn something new.
When an adult asks you for a favor, do it without grumbling and with a smile.
Related: Use this Table-Setting Map as a Guide
When someone helps you, say "thank you." That person will likely want to help you again. This is especially true with teachers!
Use eating utensils properly. If you are unsure how to do so, ask your parents to teach you or watch what adults do.
Related: Mrs. McVeigh Weighs in on Proper Utensil Use and More!
Keep a napkin on your lap; use it to wipe your mouth when necessary.
Don't reach for things at the table; ask to have them passed.
See more on teaching manners to your toddlers and preschoolers.