Most of the time you gain a corruption point by failing a corruption save. There are a few cases where you gain a point without getting to make a save (ie. the first time you cast Demonic Pact
So the next question is; "what provokes a corruption save"?
By the RAW you provoke a corruption save by making peacful contact with a corrupting influence (ie. a demon or somebody with a Corruption Score of 10+) or by being power-drained in a War of the Souls by such a corrupting influence. There are also a few other specific sources called out (such as living in an area with Green Fire Stones).
However the rules also suggest, though they do not state, that other sources of corruption may exist including using corrupting magic items and participating in unspecified corrupting behaviours. I encourage GM's to personalize their own games by calling for more or fewer corruption saves depending on the tone they want for their game.
Also, you may want to read this thread
for more info.
Hope that helps.