How do you pluck a bass?
Place your thumb on the face (not on top) of your pickup closest to the neck. Let your thumb linger just above the E-string. You will see why shortly. On most bass guitars this should put your plucking fingers in a central position between the neck and the bridge.
How hard do you pluck bass?
Ideally, you should pluck the strings medium light most of the time, and be able to have a more subtle touch or conversely to pluck harder when the song calls for it, e.g. for a breakdown or crescendo. The ability to mix and match between light and heavy attacks is an important skill.