In general the use of 'like' in this way is considered a filler and carries essentially no meaning of it's own. 'I'm like / he's like' is slang for 'I said / he said' and comes from 'Valleyspeak' a slang dialect originating in Southern California. To most native speakers this sounds very immature or teenage-ish, I would say. I wouldn't recommend anyone learning English to consciously use 'like' in this way as it would probably sound too contrived. Hope that helps : )

The first answer is true, but I think it expresses more than just 'he said, she said' because people use it to express how they felt in a situation, even if it wasn't what they said. For example, if someone said "He asked me the phone number for 911 and I was like, 'You're so stupid.'" The speaker may not have actually said 'You're so stupid,' but they thought it. It's not proper and I wouldn't recommend using it in professional settings, but I personally don't think it sounds immature or teenager-ish. I know many people, including adults who use this. I also don't think many native speakers even notice when people use it. If it's how you talk to your friends to have fun, I don't think it's a problem.

What you are describing is common in Americans of all ages - not just young people. I think it's more a bad habit then it is slang. (It really annoys me! When I hear it, I think people are speaking without thinking!) Intelligent people have this habit as well. It's not just teenage girls. But it is definitely more common among women than men, and more common with Americans than other English-speakers. I recently listened to a radio interview between two women. (Terry Gross is a radio host from Philadelphia, and she was doing the interview). For the first twenty or thirty minutes, the interviewer (Terry) kept saying "like" in every sentence. The woman being interviewed was not. But in the final ten minutes of the interview, both women were inserting "like" everywhere. ("I was, like, 'yeah.' And she was, like, 'why?' and I was totally, like, . . . " you get the idea!) It's contagious, like a disease! (Sorry. Like, I didn't mean to, like, rant. But it, like, drives me crazy!)