Even with a high TOPIK score, getting stuck ordering at a restaurant is common. That's because exam Korean and everyday Korean use different expressions.
Not enough on its own. TOPIK assesses listening, reading, and writing; speaking is a separate exam.
Exam passages center on written and explanatory style, but real conversation has a lot of contraction and omission. '뭐라고 했어요?' (What did you say?) shrinks to '뭐랬어요?,' and particles get dropped often.
It's faster to learn frequently used expressions whole, by situation. If you try to assemble every sentence from grammar rules each time, you can't keep up with the pace of conversation.
This site's '실전 한국어 표현 300' (300 Practical Korean Expressions) walks through 15 real-life situations — restaurants, hospitals, shopping — as stories, so you pick up the expressions along the way.
It's safer not to use it until the other person suggests it first. This holds even if you look about the same age.
In Korea, the usual flow is to start with polite speech when you first meet, and later, once you're closer, the older person suggests '말 놓자' (let's drop the honorifics).
In places where you're receiving a service — restaurants, shops, taxis — use polite speech even if the other person looks younger than you. Casual speech is taken as rude there.
In Korea, asking someone's age to figure out what speech level to use is common. It's generally not taken as a rude question.
That said, it depends on the setting. It's natural between people who look about the same age, but it's not typically asked in a business relationship or where the age gap is obvious.
'나이가 어떻게 되세요?' (roughly, 'How old are you?', softer phrasing) sounds gentler than '몇 살이세요?' (a more direct 'how old are you?'). For someone clearly older, use '연세가 어떻게 되세요?' (the honorific form for age).
The expressions used around ordering are fixed. Memorizing just these whole covers most situations.
To call someone over, say '여기요' or '저기요' (excuse me). To order, say '이거 주세요' (I'll have this) / '~ 하나 주세요' (one..., please). To add something, say '이거 하나 더 주세요' (one more of this, please).
For paying, use '계산할게요' (I'd like to pay) / '카드 되나요?' (Do you take cards?) / '따로 계산해 주세요' (separate checks, please). For a to-go order, say '포장해 주세요' (to go, please).
Saying it upfront at the start of a conversation gets the other person to slow down — '한국어를 잘 못해요' (My Korean isn't very good) or '천천히 말씀해 주시겠어요?' (Could you speak more slowly?).
When you didn't catch something, '다시 한 번 말씀해 주시겠어요?' (Could you say that again?) is the safest option. Just repeating '네?' (huh?) leaves the other person unsure what to repeat.
To confirm you understood, ask back with '~라는 뜻이에요?' (Does that mean...?). Rephrasing it in your own words lets you catch a misunderstanding right away.