When ordering from a tavern-keep, bluffing a city guard, parleying with an enemy, plumbing information from a (purported) beggar, assessing the motives of the queen's vizier, or cajoling your fellow adventurers to heed your advice, don't forget to roleplay!
The GM determines how social interactions are handled. Some like lots of ability checks, others like retaining control. Some like a bit of both. Regardless of approach, character identity should play a big part.
Don't forget that during combat on your turn you can make a "brief utterance" to friend or foe.
The most commonly used check is a Charisma check with the Deception, Intimidation, Performance, or Persuasion skill applicable. Use a plain Charisma check when you try to find the best person to talk to for news, rumors, and gossip, or when you try to blend into a crowd to get the sense of key topics of conversation. (82)
When you try to determine the true intentions of a creature, use the Wisdom (Insight) check. (82)