Connecting to Linux Instance using SSH
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AccessingInstancesLinux.html
ssh -i /path/my-key-pair.pem <USERNAME>@<PUBLIC DNS OF INSTANCE>
For Ubuntu Instance, username is 'ubuntu'. You can copy from your Instance List of AWS console:
ssh -i /path/my-key-pair.pem ubuntu@<PUBLIC DNS OF INSTANCE>