Actually, first, allow me to refer you to PoorCynic's workshops on characters and speech. There's a ton of good advice in there, and it'll save my fingers. ^^
Now, to elaborate a bit on what's in there: how do you determine what unique word choices to give your characters? PC touches on this a little—a coarse character may swear a lot, a sociology student will talk differently than a high-school educated person, etc. But what does that mean?
Here are three things to consider when finding your character's voice.
1. Background
Where did your character grow up? What kind of language and speech cadences did they hear around them as a child? Someone from a big city might speak more briskly, use more blunt language, and use choppier sentences than someone from the rural South, where life happens in less of a hurry and more emphasis is placed on manners than rapid conveyance of information. People from different countries, who may or may not have English as their first language, also speak with different rhythms, variant vocabulary, and possibly nonstandard grammar (following the habits of their first language).
Also, some families are more inclined to get loud with each other than others. You can convey intensity of speech with shorter, simpler sentences without having to resort to lots of italics and exclamation marks, but you can use those tools (sparingly) to set one character apart from another, too.
2. Education
This is a biggie. How much, where, and how your character was educated is possibly the biggest influence on how they speak as an adult, because like the clothes we wear, it's a huge part of how we choose to present ourselves to others—although, in moments of great stress or excitement, people tend to revert to the mannerisms they learned as a child.
A quick way to set a highly educated person apart from a less educated person is the percentage of Latin- and Greek-derived words they use. Someone with a college-level education knows more of the fancy Latinate synonyms to the simple Anglo-Saxon building blocks of English, and uses them naturally. They'll also speak with stricter grammar than someone who has never known or cared what a split infinitive is.
It's important to note here that "less educated" does not mean less intelligent. Intelligence is reflected in one's ability to process information and make good decisions about what to do with it. The more world experience you have, the easier this gets, but just because someone has lived a more isolated, less complex life does not make them stupid.
The exact kind of education your character to has will influence their way of thinking, and therefore their way of speaking, even further. That sociology student thinks and expresses themself in sociology terms. A movie director thinks in terms of cinema. A politician (theoretically) is always thinking about the impact their speech will have on people's perception of them. A self-taught entrepreneur will also have self-made speech habits, drawing from the books they've read and the people they know.
The point here is that people learn to speak in ways that most effectively convey their interests (helps us make friends), and status (we all value this to some extent), and intent (helps us get what we want) to the people around them. An educated person who has learned to communicate with similarly educated people has a different set of tools for doing this than a less educated person who has learned to communicate with less educated people.
3. Personality
This is perhaps the finest level of refinement when it comes to speech. A character's background and education give them certain tools in their linguistic toolbox. Their personality determines when and how they choose to use those tools. A selfish person might talk about themself most of the time and use the highest status speech they know to impress. A considerate person listens to and asks relevant questions of others, and leaves themself out of it. A happy, excitable person has a faster cadence and uses shorter, bubblier words than a taciturn one.
Personality determines the amount of swearing one chooses to use, whereas their background and education determine exactly how they do it.
Personal interests and hobbies can influence the figurative language a character uses. Someone who loves music may use musical metaphors and similes. Someone who loves math may explain things in terms of equations and functions. Someone who loves The Lord of the Rings may quote the books and/or films to make a point.
And so on, and so forth.
I hope that helps! Speech is complex, because it's completely wrapped up in the sociology and psychology of human beings. To really understand it, you have to understand people. It's hard, but you can get pretty far just by listening attentively to how other people talk and paying attention to how other writers do dialogue. Personally, it's my favorite thing to write, and I've put a lot of thought into it. I'm happy to talk about it more if you have more questions. ^^
~Neshomeh