The syntax for using the aws cli is as follows:
aws [options] <command> <subcommand> [parameters]
Some examples using the 'ec2' command and the 'describe-instances' subcommand:
aws ec2 describe-instances
aws ec2 describe-instances --instance-ids <your-id>
Example with a fake id:
aws ec2 describe-instances --instance-ids i-c71r246a