Arq uses Amazon Web Services for storage, so you'll need to create an Amazon account.
Amazon Web Services uses 2 "keys", an "Access Key ID" and a "Secret Access Key". To find out what the keys are for your Amazon account, go to the Security Credentials page and look for the Access Credentials section:
In the "Access Keys" tab of the table you'll see "Your Access Keys".
Here's a screen shot of the Security Credentials page showing how to find the Access Key ID and the Secret Access Key: