How to get a public transportation ride from the airports?

Hey people, I'm supposed to visit London in June and I have just started making plans, little by little. There are 2 airports where I have the option to land - Luton and Gatwick.

Is there any form of public transportation to take me from the airport to the city? Like a metro or a bus? I don't want to spend £100 on an uber. Any advice on this would be helpful. Thanks! :)