Once you've purchased your Google Apps service, the first step is to run the Setup Wizard. This will lay the initial foundation to get the account up and running, so that you will then be able to progress through to the Activation / Verification and general configuration.
To begin, login to your Crucial account, and access My Services.
Next, select the Google Apps for Work service. Note that the lack of a hostname indicates that the Setup Wizard has not been completed yet.
Next, simply input the domain you intend to use with Google Apps.
Once setup, it is not possible to change the primary domain of your Google Apps account. If you're planning to use a different primary domain in the future, or you have multiple domains, do not rush this step - consider carefully which domain you provide here.
On the next screen, simply fill out the details as required.
Note that the initial user you create here will be the administrator account for your Google Apps service, and will have access to make changes on your Google Apps service that regular users cannot do. Don't worry about additional users for now - those will be created later.
You're done! With the initial setup complete, you will be taken to the general service page.
From here, you will want to progress to the Activation Steps, which is required before you can use the service. See this article for more information.