You can't change the email address, only the display name as mentioned. It sounds like this isn't what you want though - you want a completely new Google Account.
You would need to create the new account and then move the data, email and other Google services, over into the new account (see below)
On the old email, you could then add an auto-responder letting people know to use the new email, forward mail from the old account into the new one, or have the new email account check the old email account.
Archived info from http://knol.google.com/k/changing-your-gmail-username-or-email-address
It is not possible to change your username in gmail. The username is
chosen at the time the account is created and is the permanent
identifier of the account.
But if you need to use a different username, it is possible. You
simply need to create a new account and move your content over from
the old account. Here's how.
Step 1: Create a new account You should sign out from your existing
account and then choose Create an account from the gmail login screen.
Step 2: Grab your mail from the old account Login to the new account
you just created, go to the settings link in the top right, and choose
the Accounts and Import tab. Gmail doesn't let you use its standard
import feature to transfer from one account to another, so you'll need
to use the Check mail using POP3 entry. Enter the following
configuration:
Username: Your old gmail email address Password: Password for your old
gmail account POP server: pop.gmail.com Port: 995 Always use secure
connection: checked Label incoming messages: checked
This will pull over all your old mail and will continue to pull over
new mail received at the old account until you disable it. It will
not, however, save any label information. You will need to relabel
your emails (or simply rely on gmail's search features to find
messages).
If you absolutely need the old label structure, the only solution is
to use an IMAP client to connect to both the old and new account and
copy over folders/labels. But this solution can be tricky, because you
can only copy over a certain (unspecified) amount of mail at a time.
The non-Google program Gmail Backup might be able to assist with this
process, but I can't vouch for its effectiveness.
Step 3: Export and import your contacts Login to the old account and
follow the instructions on the Export Gmail contacts page. Remember
the location of the saved file.
Login to the new account and follow the instructions on the Importing
CSV files page using the file you just saved.
Step 4: Export and import your filters If you have many filters on the
old account, you can use the Filter import/export feature of gmail
labs to bring them to your new account.
Step 5: Non-Gmail Google account contents If you have other content in
your Google account that you want to transfer (calendars, documents,
photos, etc), you'll have to do this separately. The Data Liberation
Front website has detailed information on how to export and import
data from pretty-much every Google product.