"Slang" is an uncountable noun.  e.g. Some people use slang.  I don't like slang.

If you need to refer to an individual instance of slang, then the usual countable noun phrase is "slang expression."

You often need a lot of context and exposure to the language to use slang well.  Of course, people hear expressions like "hey bro'" a lot, so this is easier slang to pick up. If you don't live in an English-speaking country, my advice is not to make learning slang a special priority. 

I think you should speak in a way that you feel comfortable! But remember that slang expressions vary greatly between different English speakers. For example, "Hey bro" is an American expression (probably inner-city like New York??) You would never hear it in the UK - "alright, mate" is probably the equivalent!

I'd never be offended by someone using slang but sometimes it sounds a bit odd when used in the wrong context. Perhaps pay attention to how the other person (native speaker) is talking and respond accordingly.

(I'm not a teacher). It's my belief that a language learner should strive, above all, for clarity. Choose words and sentences that are so simple and clear that a native speaker will not have to work hard to understand you. You are certain to have an accent; that makes you a little harder to understand. Compensate for it by being especially clear and simple in your language. 

Slang doesn't usually help. There are several problems with it. I don't know if this is a cultural universal, but in English-speaking countries it is always funny (and embarrassing to parents) when their children start to pick up "bad words," which starts to happen at age seven, eight, nine. The problem is that children do not have the right social sense to know what can be said, to whom, in what circumstances. 

Slang expressions usually have a fairly precise rhythm and cadence and if you don't get it exactly right, the native speaker actually doesn't understand the words you've said. Then he hears them, but he hears them in their literal (and illogical) sense. 

Why is he talking about Monday morning? Does he have an appointment Monday morning? What is that, why does he want water back, is he talking about someone named Walter, is he saying he needs a quarter back? Monday, morning, quarterback... is he on an amateur team and going to be playing--Oh! Of course! "Monday-morning quarterback."

There's also the danger that you may apply grammar rules and "rewrite" an idiom or slang expression without knowing that you are doing it (as in the movie "The Russians are Coming!" where the Russians keep saying "the coastal guardians," which is quite puzzling to hear, because to them it is logically equivalent to "the Coast Guard," so why should it matter which you say?

By all means, speak to people as they speak to you, but don't try hard to add idioms and slang to your active vocabulary.