You can create an AMI from either an EBS or S3-backed running instance. The simplest method is to use the AWS Management Console to select the instance and click on 'Create Image' from the Instance Actions menu.
This will create either an EBS or S3-backed AMI, counting on the type of instance.
Be aware that creating an AMI from a Running instance is inherently potentially unsafe - the storage isn't imaged as a hard 'point-in-time' snapshot, therefore changes to the filesystem whilst the image are being created might not be recorded, and will even end in a compromised image. You should quiesce the OS as far as possible before beginning.
Note additionally that your instance will reboot throughout the image-creation process, therefore make certain you are prepared for any temporary loss of service from the VM.