Ok, here's what I would do in your shoes...
The store did you wrong when they entered a Gmail address that is not yours. Your phone was registered to that Google account.
I assume since you just go the phone you haven't installed lotsa apps, taken bunches of photos, etc. So I would do a factory reset, which puts the phone back to out-of-the-box condition and gives you a fresh start. It just takes pushing a combination of the power and volume buttons. It varies by device but the user manual will have it. Then before doing anything else with the phone get a Google account (aka Gmail address) from here:
https://support.google.com/mail/answer/56256?hl=en
There's some links to basic questions on the page. When ready just click on 'Create an account'. You do not have to use this account for email unless you want, but you need a Google account to setup the phone to use the Play Store for apps, music, etc. You should save your contacts to your Google account for online backup and easy restoration when you change phones. Your Google account will also keep track of apps you install and restore them, some settings, etc. Very handy and might save your butt.
Your phone has 2 email apps installed - a Gmail app and a standard email app. If your phone has the newest version Gmail app it will handle any email account. If it has the older version Gmail app, that is Gmail-capable only. I don't know which version you have, but you may be able to update it in the Play Store, if not now then soon. However, the standard email app will handle any email account, Gmail or not.
Either email app will automatically configure your Gmail account. Unfortunately, Netzero is still stuck in the last century and does not offer IMAP email service, only POP3, so you will have to enter the configuration for that account manually. Netzero's instructions are here:
http://help.netzero.net/support/webmail/mobile/droid.html
Hope this helps a little. Post back here with questions.