The Doctor's TARDIS was a Type 40 TARDIS, which he stole when he lived back on Gallifrey. He used it to travel across the universe and through time. When arriving in 1963, Earth, the Doctor discovered that the Chameleon Circuit had broken rendering the TARDIS unchangeable and so its main image remained as a Police Box from the 1960s.
When arriving in 1963, the Doctor hid the TARDIS in 76 Totter's Lane, a junkyard, where no one would find it. However, when Susan's schoolteachers came looking for her, they came across it. The Doctor was forced to allow them inside and they discovered that it was bigger on the inside. The Doctor took them away from 1963 and the TARDIS travelled through time. It arrived in Paleolithic times and provided an image of the outside on a scanner. After leaving the TARDIS they didn't return for a while until they finally escaped the clutches of the Tribe of Gum and then travelled once again. (An Unearthly Child)
The Third Doctor removed the TARDIS console from his TARDIS in order to experiment on its controls and hopefully rectify his exile. However, the non-protection meant that the time rotor was faulty and it sent him and his companion, Elizabeth Shaw forwards in time. (The Ambassadors of Death)
This feature ensured that only the Doctor or a recognised individual like a companion or close friend could use the controls. Also, for some time, the TARDIS door key needed to be used by someone recognisable.