Google CodeJam is an online programming competition that takes place each year. You must be 13 years old in order to compete, and you need to be 18 years old to attend the onsite finals (if you were to qualify for said finals).
Registration usually begins in March, and the qualification round usually occurs in April. Check the Google CodeJam schedule page for more details.
Explore the Practice and Learn section of the CodeJam website to get yourself up to speed before the competition begins.