First, make sure you're on the Settings tab in SiteMail. If not, click Settings along the top of the interface, and then Webmail Settings.
Account Name: your e-mail address
Real Name: the name that appears on outgoing e-mail messages
Reply To: if your reply-to address is different than your account address, then specify it here
Thread Support: turn it off if you prefer to list all e-mail messages individually and not collected into threads
Thread Limit: if Thread Support is on, then specify the time period for messages to be included in a given thread
Default View: if you prefer 3-pane view, then select 3 Pane here so that you don't have to set it manually every time you log in
Interface Language: select your preferred language for the interface
Messages per page: select the number of e-mail messages to display per page
Timezone: select your time zone
Signature: if you would like to include a signature in all of your outgoing e-mail messages, then enter it here
After any change, be sure to click Save Settings.