The farthest the Doctor has ever travelled in the TARDIS is to the Big Bang (the beginning of the Universe) and 100 trillion years into the future (the end of the Universe) (in the episode "Utopia"). The inside of the TARDIS is much bigger than its outside (it is "dimensionally transcendental"). It has a lot of different rooms. The most important of which is the "console room" where the Doctor pilots the TARDIS.