Basically, a specific panel in the Tardis opens up a GUI for the forcefield. There would be several options for the forcefield here, but the basic idea is, it runs on Zeiton-7. It's like a furnace, slightly slow running,  and the forcefield stays up until disabled or when the Zeiton-7 runs out. Now, it would be unpleasant for you to be laughing at the daleks trying to shoot you when the forcefield unexpectedly disappears, so it would message you a warning when the forcefield is almost out of power.

Another mechanic would be the ability to be shot down by enemies and annoying players that you can't remember why you invited them onto your world. When the Tardis is shot down, it would be unable to fly or teleport, leaving you stranded on a distant planet, unless of course you were shot down immediately after building the Tardis, which is lame. A forcefield would help prevent this, but every shot it takes removes a small percentage from the Zeiton-7's power, so you're only in danger from really persistent mobs and players. If you ARE stranded, you're dead, unless you can repair the Tardis using a thing, but that's straying from the main topic here.