The easiest way is to pick a couple of one sentence motivations likely to affect your decision making for the character, and make those motivations different from ones you personally have. Make decisions for your character in the fantasy (i.e. imagined) environment, keeping those motivations in mind when they're applicable.

As stuff happens in game, you can add or replace motivations based on that stuff. It'll get easier and more instinctual as you go.

A bunch of TRPGs have this system, with mechanical impact from heavy to effectively none. Included among them are Exalted, Torchbearer, Powered by the Apocalype, Mutant Zero/Forbiddan Lands, D&D 5e. It works well, because it gets to the core of what roleplaying is, making decisions for your character in the fantasy environment.

The important thing to remember is roleplaying is not just talks time / voice acting / emoting. Although those things can be great if Done Right TM*

Done Right TM may vary from table to table. No assumption that any given reader of this post is Doing it Wrong TM implicit in this statement.