Go to Brixton station.
Tap your contactless debit card on the reader by the entrance barrier. (Using a contactless debit card is MUCH cheaper than buying a paper ticket).
Brixton is at the very end of the Victoria Line and is not served by any other lines. Therefore trains only go in one direction from there, so you can't possibly go wrong when finding your way onto a train.
Stay on the train for 5 stops, alighting at Oxford Circus.
Follow the signs for 'Central Line'.
When you're faced with the choice between Eastbound and Westbound services, make sure that you go to the Westbound platform.
Once you're on that platform, all trains are going where you want to go. (i.e. you've not got the complications of having different lines running from the same platform).
Take a train from there, for 6 stops, to Holland Park.
At Holland Park, tap your contactless debit card on the reader by the exit barrier.
To answer your original question though, ticket offices are being phased out on both National Rail stations and London Underground ones, in favour of payments by app, contactless debit cards and paper tickets dispensed by machines.
However London Underground stations are always staffed when services are running.