You’re actually right! The books used in schools very rarely use slang, whereas people going about their daily live do. Slang, which changes a lot from country to country , is very common in informal spoken language. And it can be impossible to understand without the help of someone who already knows what it means.
Take the title of this post as an example. Rattlesnakes and cobras are very deadly snakes, deadly in the sense of ‘make you dead’! However, as far as I know slang can’t kill.
‘Deadly’, in Irish and Australian slang (and possibly other countries, too), means great, very very good, or excellent.
So, how can you prepare yourself for this? Well, lots of reading for a start. Pop magazines and websites would be a good place to start. But if you have some time, you can find some really nice posts on blogs that will explain some of the more common expressions to you.
I found one such blog, with some really nice examples and have reposted it here for you. If you have any questions, give me a shout!
Link: Slang Post