An atheist is a person who does not believe in any gods. Notice that this definition does not require an atheist to positively "believe that there is no god", although many atheists believe this as well. People who believe that there is no god are sometimes called "strong atheists" and people who merely lack a belief in god are sometimes called "weak atheists" or merely "agnostic atheists."
Apart from not believing in any gods, there is no official atheist doctrine. There is no atheist pope, no atheist church, and no atheist rules. This does not mean that atheists do not also follow societal and legal rules, but it does mean that there is nothing specific about atheism that tells you how you should live. However, there are comprehensive philosophical positions that include atheism as a part of the overall philosophy. Secular humanism is the most well-known example.
See also: Atheist vs. Agnostic