Someone asked me this on pm and I thought I'd share, I'm not an expert feel free to correct me or list your suggestion. I've learned this from observing dramas and korean shows, and a bit of research.

unnie= if you're a girl you use this to address older girls, it means older sister, close friends and siblings use this term
noona= if you're a boy you use this for older girls, also used between siblings, close friends
dongsaeng= is not really gender specific and they use them to call a younger sibling or a younger close friend
oppa= this means boyfriend or older brother and only girls use this, for sibslings/close friends
Hyung= older brother, used by boys to address their older brother or a friend older than them
the terms I had mentioned above are used if the relationship are close enough, koreans don't use this if they had just met. If they are not close you add -ssi to the name
Like Kim Jong kook-ssi, they normally use the whole name, if you drop the surname that's like getting a step closer and once you start getting close you call them with oppa/noona/hyung/unnie like Jong kook oppa or just oppa

koreans are very big on age and seniority so the terms are important to them. if you are the same age with someone you can use informal speech and call them by name, that means you are chingu or friends, it's hard to explain the use of friend in that context but it's like Gwangsoo and Joongki's "same-age friends" concept. If you're with new acquaintances you'd normally go for the one with the same age with you. maknae means the youngest and in their culture they are usually the one who does the hard work for their elders, that's also why gwangsoo gets 'bullied' but the elders always takes care of the youngest, it's normal and I wouldn't really call it bullying


Appa/Abeoji means father... I noticed the younger children use appa more and abeoji for older ones but I think abeoji is just more formal than appa
omma/ommeoni means mother and like abeoji ommeoni is just more formal and if you use that it will seem like the child/mother is not too close
Ajussi is uncle, but you can also use this to address older men, coz oppa is not cute anymore if the man is your father's age lol also used for strangers coz it will be weird if you call them "sii" without a name
Ajumma is the opposite of ajussi, use this for older women.
halmeoni is grandmother
harabeoji is grandpa
samchon also means uncle from father's side or he's a close family friend or close to you like a real uncle
gomo is aunt from your father's side, "imo" from mother's side and you're also blood related, but same with samchon, also used for close family-like people
sunbae= this is for your senior, someone in school/work/etc. who came before you, like sempai. Some people might have higher position but you can still be the sunbae so even if they have more power they still need to respect you, 'hoobae' is the opposite of sunbae, this means you're the junior

anyeohaseyo=hello, I seem to notice this is also good morning/afternoon/evening it's like always their greetings hahaha
mianhe= informal 'sorry' 
gomawo is informal 'thank you' kamsahamnida or gomapsumnida are formal ways to say this
aniyo/ani= no
neh/deh= yes
saranghae = I love you
choa= like
naega (I) choa = I like you, naega choayo = I like you -- I'm not korean so I am not sure how to explain this lol plus the romanization and spelling is hard...

and if you want to be respectful you add -nim in the title like (this are the words I had heard, not sure how others can be used with nim)
hyungnim
noonim
abeonim
sunbaenim
omeonim
PDnim

you add -yo or -nida for more formal korean words ike kamsamnida, saranghaeyo, gomawoyo -- you might find it strange that an already informal word is made formal with 'yo', but at least it's not like the ultimate most formal term, something like that. I'm not sure anymore about the others but for starters that's it

I know more phrases and stuff but this is the most common I see plus not everyone is well versed in korean so it's not always advisable to use korean terms unless it's for addressing people... if you search there are very complicated stuff about relationships and how you call them.. I doubt people had read about those too so it's better to stick to basics.
 

Again this are from watching dramas and I am not sure of the others... this was not the first time someone asked me so I'm posting this here for the newbies. Feel free to ask or answer if ever :)