In ME1, the Paragon meter needs to be at least 75% full to unlock all Charm ranks, at which point it's just a matter of investing the necessary points (though keep in mind that between Spectre induction and reaching certain milestones in either morality, up to 4 of those ranks can be awarded for free).
In ME2, there is no Charm skill; the availability of Charm options is calculated based on how many of the Paragon points that you could have earned up to that point were actually earned; harder Charm checks require greater percentages of Paragon points.
In ME3, both Paragon and Renegade points contribute to Shepard's Reputation, which in turn determines the availability of Charm/Intimidate options. In this system, you could go full Renegade through the entire game and still be able to Charm TIM into killing himself. Complete side jobs, talk to your squadmates and other NPCs, and just progress through the game and you should have no problem getting a high Reputation score. Terminator-HIX (talk)