All secret should be set via ENV or in files named .auto.tfvars
which will be excluded from version control.
You need to setup aws profile with name "terraform"
This configuration uses aws s3 bucket to store, first thing to do:
# Create file with secrets `.auto.tfvars`
# Inside ./terraform/backend
terraform init
# If backend bucket already exists this will be no-op run
terraform plan
terraform apply
Due to terraform limitations you can't use variables inside backend configuration
# Inside ./terraform
terraform init
terraform plan
terraform apply