Talk in a Mature Manner

Here is how to talk in a very mature manner.

Steps

Work on your tone. Do not talk too loud because it will be taken that you have no proper upbringing. However, do not talk to lightly or it will be taken you are shy. Talk in a medium voice, so the person you are talking to can hear you, but not the person across the room.
Speak from diaphragm. It is rather hard to explain, but when you talk to not to talk through your nose. Try to talk from your stomach. If you are doing it right, you'll notice that your voice is deeper or more mature sounding.
Know when and when not to speak. Do not interrupt anyone unless it is an emergency. But do offer small talk when can be done. School is always good to talk about to adults. But not the school drama of who likes who. Talk about your "studies" and current events.
Increase your vocabulary. However do not use long words just to sound smart. Use words that are synonyms of other words. Such as instead of love say affection and small changes like that.
Always use the correct title when addressing someone. For instance, say "Mr.", for a married or unmarried man, or "Mrs." for a married woman. "Ms." works for both married or unmarried women.
Spell words correctly while chatting or posting online. Check over a post before you post it to make sure there are no spelling errors.
Be sincere. Don't fake your manner of speech to impress others.
Avoid laughing or joke inappropriately. If someone is trying to be serious with you, and you try to be humorous, you will not be considered mature.