Start from the very beginning. An Unearthly Child episode 1, and don't skip a single episode. Even the "missing" ones. They all exist in audio form with fanmade reconstructions if you have ADHD and need visuals. If you argue otherwise or skip anything you are a pussy. Continue until you reach Survival episode 3, which is the end of the original show. From there you can peace out or watch the 1996 TV movie and Scream of the Shalka as bonus material. If you're feeling adventurous then "watch" Death Comes to Time as well. After that, STOP. Nothing else matters, but you can listen to some of the audios by Big Finish if their premises sound interesting I guess.
NuWho is fanfiction and doesn't matter.