Sometimes you need to do actions as root to configure things for your AMI.

SSH login user

Firstly login as as the regular user for your AMI instance.

Username: ubuntu
or
Username: centos

No password is used. You should connect to your instance using the public IP address and your public key

ssh -i mykey.pem ubuntu@public-ip
or
ssh -i mykey.pem centos@public-ip

Becoming a super user

There is no root login for security reasons. If you want to do things as root then you can use the sudo command once you are logged in as the user ubuntu or centos, for example:

sudo ls /etc
sudo rm /tmp/test.file

and even give yourself a root shell by doing

sudo bash

Be very careful with root access as it has total access to the system and you could end up deleting an important file by mistake.

Changing the file ownership

If you create files or directories as root you will out likely need to change the ownership to the same user and group as the rest of the files in that same directory (for example www-data).

chown www-data:www-data my-file