One of the most important talents are related to persuasion. These are charm and intimidate. There's no point investing in both, because their effect is similar. The difference is how Shepard brings other characters round.
Chosen talent should be improved as fast as possible. It's best to concentrate at the beginning on its development and prioritize it while gaining next experience levels. This way you could get a lot of additional experience and morality points. It happens sometimes that only thanks to persuasion you could avoid unnecessary bloodshed. Remember also, that completing a quest using charm/intimidate is usually rewarded better than doing it in a different way.
Statements which use persuasion are very easy to recognize. The charm ones are blue, the intimidate ones are red. Usually to get to use them, you must first choose statements situated on the left of the conversation wheel.
Charm/intimidate at maximum level is required to alter major game plot direction, which will unlock an achievement (Charismatic).
Charm/intimidate at higher level brings also additional, material gains - with the first you get discounts at shops, with the second you could sell your objects at better price.
Hint! If you choose to play this same character twice, as paragon and as renegade, it's good to spend persuasion points in a different way each time. Invest in charm first and at two third of game begin to develop intimidate. Use charm to the end of the game, but slowly improve intimidate. This way when you start the game the second time, intimidate will be developed and could be maximized. Remember, that with every experience level amount of points to spend on skills decreases. That's why advancing character while playing second time is really time-consuming and demanding.