Yeshivish exists most strongly in Yeshivish communities (whodathunkit?), but many orthodox Jews (primarily Ashkenazim) keep some yeshivish up their sleeve. Sometimes, it's just a faster way to communicate an idea. The words may be either Yiddish or Hebrew (generally not modern Hebrew), and they are sprinkled liberally throughout the English conversation/shiur.
For learning yeshivish words and phrases, I suggest listening to lots and lots of shiurim. (Listening to them will be good for you, regardless.) In the shiurs, rabbis will often use yeshivish, but kiruv shiurim will usually also give an English translation of the phrase. That way, you get both the meaning and how to use it in a sentence naturally. In many (American?) communities, a little yeshivish is the key to "passing" well. You walk the walk, so now you must talk the talk. Or at least understand other people talking.
I've cultivated a working fluency in yeshivish, but I try not to speak it as a general rule. I don't trust myself to choose both the correct word/pronunciation and right meaning in the same sentence. However, I understand most yeshivish spoken to me. What I don't understand is often clear from the context of the conversation, but there is no shame in asking someone to explain a word or phrase to you.
How much yeshivish do you understand? If you can understand this video, you are just fine. You will not understand everything, but if you understand what's going on, then you can survive just about anything. (The video is a response to this video, but you don't need to watch it to understand the video below.)