You will need to make sure that you have your AWS credentials stored in
~/.aws/credentials. This will most likely look something like
[default]
aws_access_key_id = AKID1234567890
aws_secret_access_key = MY-SECRET-KEY
If you have installed the command line interface, you can configure your system easily using
$ aws configure
More details with regard to this can be found here.
Verify your credentials by listing your running EC2 instances using
aws ec2 describe-instances