How far is Golders Green to Waterloo, and what number bus do you catch?

You can get the Northern Line tube from Golders Green to Waterloo, just make sure to catch the train heading towards Kennington via Charring Cross as the Northern Line also splits into the Bank branch which is no good for Waterloo.

Tube is the fastest way to make this journey.

However, if you're intent on taking the bus, then you'll have to catch the A6 to Victoria, change at Baker Street, and then get the 139 to Waterloo.

Total journey time should be around 35mins by bus, and for future reference you can plan all your journeys around London using the TFL Journey planner here: http://journeyplanner.tfl.gov.uk/