Here is a common spoken greeting among teenagers: "S'up!" When two teenager boys meet, one might say, "S'up, bro!" Or two girls might say, "S'up, Girl!" or "Sup, Hon!" To which the answer is "S'up!" It's a contraction of "What's up?" but is not treated as a question, but as a sound of greeting.

Slang differs widely between English-speaking countries. Mass-media has made most of us aware of other countries' slang (the "s'up!" is a great example, thanks!), however if an American or Irish ended their sentences in "mate" (as we do in Australia), it would sound so very weird. I think the solution is to pick which region you prefer, eg. maybe where your friends are, and focus on that. It can be slightly offensive if you start using (for example) American slang when talking to people from the UK... and vice versa. Have a look at blogs and see how they're written (keep in mind where the author is from)... www.urbandictionary.com is a great guide to modern slang.

Slang often sounds silly unless used by a native speaker. You'd be laughing if a foreigner attempted to use slang in your language whilst talking to you. Think about it,

I don't think there's any slang that applies to all native English speakers. That's why it's considered slang, and not standard. Instead of slang, I would suggest focusing on learning idioms, which are similar to slang but are generally standard. Here's some examples: A steal -- If something is a steal, it is for sale for less than its value. "That car was a steal, I only paid $500 for it!" Axe to grind -- A grievance, issue, grudge or resentment one person holds against another. "He didn't promote her because he has an axe to grind." A huge list of English idioms: http://www.usingenglish.com/reference/idioms/