When you have launched your new instance and made it available for ssh you want to login.
The username is different depending on the Linux flavor you are using but you will always use the keypair and not a password.
This is the ssh command you will use is you login from a Linux client.
# ssh -l UserName -i /path/to/my-keypair.pem 126.96.36.199
Here is a list of the user names for each Linux dist.
When using Heat to deploy instances the user name will be ec2-user instead.