You need to set up your device to use Google Play: from their help page...
To use Google Play to download Android apps, you'll need to have a Google account with a Google email (either Gmail or a Google-hosted domain) associated with your phone or tablet. Follow these steps to add an existing Google account to your device or create a new Google account for use with Google Play:
Turn on your device and tap the Google Play Store app.
If you haven't added an email account to your device, you'll be prompted to "Add a Google Account."
Follow the instructions on the device, which will walk you through signing into an existing Google account or creating a new Google account.
Once you have signed into your existing Google account or created a new account, you'll be able to use Google Play to download Android apps.
Keep in mind that if you intend to purchase apps, you'll also need to make sure your Google account is set up for use with Google Wallet.