Get your copy of a TV season. You might want teen drama or detective drama. Not sure about the titles? Look up "teen drama" etc on Wikipedia. Your decision depends on what age group you want to sound like. Also make sure that the season is recent. A series that is 10 years old may not sound like how people actually talk now. Watch the series on your computer.

Then you want to get a copy of the TV transcript for whatever episode you are watching, so that you can speak along. Google is your best friend and there are plenty of free TV transcripts done by fans.

Make the copy into Words document format. Then as you watch the episode, understand how the character is feeling and try to mimic the speech. If you cannot pronounce the word exactly as it is in the episide, you can look up an online dictionary for its pronunciation key. I suggest YourDictionary.com. Then copy and paste the pronunciation key next to the word in the Word document. You want the pronunciation key to be encased in parentheses or brackets.