Don't worry about speaking 'cool' or 'hip' or modish or slang English... not much English is outdated (except those in Chinese-English dictionaries 哈哈, and the newest and coolest of words are (of course) known only to the part of the population that 'invents' the phrases. Clearly the Internet and social media will be the birthplace of new phrases, but many of them will appear and disappear and have only the briefest vogue,

Hi Wen Yan, Watching TV shows or movies is useful, but can be too fast. Reading newspapers or websites are useful. I think internet Blogs might be very useful. People often write them in ways that they would talk, and you can take the time to understand them because they are on screen. Most english is the same now as it was 100 years ago. These days the most common changes I encounter are the internet words such as, "LOL, (laugh out loud)" and many other phrases used in texting. Also, every since 30 years ago. People add the word "like", "so", and "he goes" or "she goes" instead of "he said", and "she said". These are terrible, but they are common - don't talk to a person who you should show respect to in that manner. Finally you can say, "I know right!" When you agree with someone if you have experienced what they are talking about (usually excited, or frustrated).

I find that the easiest way to learn some colloquial terminology within your target language (for you, English) is to start speaking it with people (and reading) and picking them up as you do so. One example off the top of my head is the word "ain't" which is an informal contraction for just about everything (is not, are not, am not): "Say it ain't so," "I ain't telling," "They ain't seen nothing yet," etc. I don't particularly recommend using it actively, though, as it is very informal and sometimes associated with being uneducated. However, it's good to know what it is in case someone else uses it.