Use the below command to create AMI of an EC2 Instance. Replace instanceid and name values accordingly.
aws ec2 create-image --instance-id INSTANCEID --name "image-of-server"
By default, EC2 Instance is shutdown to capture AMI to gaurantee a clean image. However if you do not want the server to reboot, you may specify the no-reboot option as shown below:
aws ec2 create-image --instance-id INSTANCEID --name "image-of-server" --no-reboot
You may optionally provide a value for description as:
aws ec2 create-image --instance-id INSTANCEID --name "image-of-server" --no-reboot --description "Monthly AMI"