Beyond the general info such as correct NAP and top keywords, you will need to provide access to your GMB listing and report any existing social accounts or citations. For a full list of info that you will need from your client, check out the GMB tab of this form: https://docs.google.com/spreadsheets/d/1u3F7s7aL8zQhfVeDEUs3ZVtRMrbFQPV3oZWJLr9XI0k/edit?usp=sharing