While your instance is running, Amazon will happily charge you on a per-hour basis – check out the pricing for more information. In general, you will want to shut down your instance when you’re done with it; to do that, go to your EC2 console and find your running instances (in green).
Then, select one or all of them, and go to the ‘Actions...’ menu, and then select ‘Terminate’. Agree.
After a minute or two, the console should show the instance as “terminated”.