in their daily life they want to speak fast. so it's very common to use contractions. i've all learnt the likes of "I'm", "you're", etc., but there are some really informal ones that are not taught at school.
that doesn't mean that they are rude, or that you can't come across them. in fact, you can always find loads of them in lyrics.
some common ones include:
gonna / gotta / wanna / gimme...
http://www.englishclub.com/vocabulary/contractions-informal.htm
http://constructionenglish.net/?p=87
you can take a look at the above websites. just remember NOT TO USE THEM in exams as they are not accepted.