In my experience, these are my most useful phrases:
이거 주세요 (igeo juseyo) – “This, please.” - Super useful in shops or restaurants when pointing at something you want to buy or order. Or you can just use “noun + 주세요”
이거 얼마예요? (igeo eolmayeyo?) – “How much is this?” (of course you’ll need to know your numbers too!)
화장실 어디에 있어요? (hwajangsil eodie isseoyo?) – “Where is the bathroom?”
Also try to learn some directional phrases early on. The Seoul metro can be a challenge to navigate initially, so being able to ask for directions helps a LOT and is something I wish I’d picked up before my first trip to Korea!
This post might be helpful in teaching you some Korean language essentials:
This one is suuuuper useful, I would use it every day especially when ordering coffee or lunch! The basics for me were:
안녕하세요 - hello
감사하다 - thank you
I think it is also important to learn to recognise sentences that might be said to you:
잠시만요 - wait a moment
어서오세요 - welcome (when entering a shop, restaurant)
포장 - take out (food, coffee)
I do not speak Korean so this is only what I can remember but if someone could translate sentences like the following that would be nice! Like not to get too confused at the shop for example:
That’s amazing you know and remember these ones when you don’t even speak the language Marine!!!
For the ones you asked, I hope it’s gonna be helpful:
Do you need a bag?: 봉투는 필요하실까요?/ 봉투 필요하세요? (봉투: bag or they even use Konglish 쇼핑백 shopping bag for this at some place)
Do you have the membership?: 적립하세요?/ 적립 도와 드릴까요?/ ㅇㅇ멤버십 있으신가요?
Do you need a receipt?: 영수증 필요하세요?
For here or take our?: 드시고 가세요? (means: Are you gonna have the food here?, you can either say 네 먹고 갈게요 Yes, I will have the food here or 아니요 포장해주세요 Nope, I’m gonna do take out)
Hope this will be helpful