First go to your device Settings and find the "Accounts & sync" option. At the bottom of this screen will be an area that shows the account(s) registered with that device. Tap the "Add account" button at the very bottom, selecting the type of account on the next screen. If you are adding an additional Gmail account, you can do this through the Gmail app settings instead.
Finally enter your account information when prompted and select the sync options you require. That's it! You now have more email accounts on your device!