There are 3 main formality levels Generally, you just drop the formal general ending and get the informal version. E.g. 공부를 하고 있다 ~ dictionary form 공부를 하고 있습니다. Most formal. 공부를 하고 있어요. General formal. 공부를 하고 있어. Informal. The 야 you keep hearing comes from the verb 이다 (to be). E.g. 학생이다~ dictionary form 학생입니다. Most formal. 학생이에요. General formal. 학생이야. Informal. The -지, -네, -거든, -군/구나,... come from special endings that are added to verbs to add flavor to speech. -지요(죠), 네요, 거든요, 군요 also exist so once you learn them you'll know how to form the informal version (generally just drop -요). -군 is more manly, 구나 more feminine. Overall, informal speech is formed by dropping 요, except you add 야 to 이다 and some other minor exceptions. That being said there is a lot more to know about proper informal speech, you need to use the right vocabulary etc. as well. Hope this helps, if you have more questions just ask away.