When your new android phone was set up, you will have added (or created a new) gmail account. Your phone is then tied to that account. It is also possible to receive emails from non-gmail accounts too, however a gmail account is still required for the market and so on, so you definitely have one of those.
If you bought the phone used, it's possible you didn't do this yourself however. You can get the email address by going to Settings >> Accounts & Sync, which should show the gmail account. (However, I would recommend you factory reset the phone, wiping all data, and then add your own gmail account upon the initial setup).