1. Access the Settings menu and then choose Accounts & sync.
2. At the bottom of this screen, there will be an area showing the current accounts associated with the phone. Press the Add account button at the bottom and select the type of account.
Note: Steps 1 and 2 can be performed through the Gmail application settings, but only if a Gmail account is being added. Other account types will need to be added through the general Settings menu on the phone.
3. Provide the account information and sign in. Different account types may provide different settings after signing in. For instance, Gmail accounts will ask what other Google services you want to sync on the phone. You don't have to choose to sync any if you prefer to check each service manually (and perhaps save a bit of battery life).
Note: If you change your mind about syncing certain services, you can always go back to the Accounts & sync area and adjust them later.
Now you can add other accounts supported by your phone by repeating the above steps.