On the left door (as seen from the outside) is the standard notice on a police box:
FOR USE OF
ADVICE & ASSISTANCE
OFFICERS & CARS
RESPOND TO ALL CALLS
PULL TO OPEN
Note: It was the last line of this notice that the TARDIS, in the body of Idris, was referring to when she accused the Doctor of opening the doors the wrong way for 700 years ("The Doctor's Wife").
When first seen in "An Unearthly Child" and for some time thereafter, there was a rather faded "St John Ambulance" badge on the other door but the badge disappeared during (I think) the 2nd Doctor's era. It reappeared, brighter and fresher than it had ever been, when the TARDIS rebuilt herself during "The Eleventh Hour". When the TARDIS subsequently rebuilt herself in the interval between the end of "Twice Upon a Time" & "The Ghost Monument", the badge disappeared again.
It has been pointed out that some versions of the TARDIS seen during the original series occasionally had slight variants on this wording. Plus, the conclusion of the episode "Turn Left" had all the wording replaced temporarily with "BAD WOLF".