Instructions
Hold down the "Guide" button on the Xbox 360 controller. This turns the console on.
Tap the "Guide" button when the console powers on. Select "Settings."
Select "Preferences" and "Online Status." "Online" is the default status, and allows all your friends to see that you are available when signed in to Xbox LIVE.
Change the setting to "Away" or "Busy" if you want your friends to know you are online, but not currently available. Select "Offline" if you do not want anyone to know you are using Xbox LIVE.
Press "B" to back out of the menu and save your settings. Scroll left and select "Profiles." Select your profile to sign in.