Hi Heather,
The key used for SSH will be the one selected during the AMI configuration process. On the Launch this Software page, near the bottom you’ll see the Key Pair Settings option. You’ll need to select your Key Pair or create a key pair and then select it from this location.
Then make sure your ssh program is set to use the key pair file for authentication. Username is ec2-user as Karel confirmed.
Best regards,
Andrew