1. Go Back To Where It All Began
If you haven't ever seen "An Unearthly Child", do so. I'm not going to lie; it isn't the best piece of TV ever made, and in some ways, it has aged quite poorly. But it is well-acted, well-written and quite ably directed. I do suggest stopping after the first part, however, since the bit where they go back to caveman days isn't really worth seeing. But the first part, where we meet the Doctor and Susan, really is. I will say that the Doctor is quite a bit different from what you're accustomed to if you've only seen the new series, and the story does suffer from a bit of first episode weirdness, like Susan saying she coined the word TARDIS. Once you're done with that, go back and rewatch as much old Who as you can. Check out things like "Tomb of the Cybermen", "Genesis of the Daleks", "Caves of Androzani" and the like. Learn about where the series got its start and then sit back and enjoy all fifty years of magnificence.