对不起，没关系
Sorry — it's fine

对不起。没关系。

Learn these two as one unit: an apology and the answer to it. 对不起 duìbuqǐ covers 'sorry' and 'excuse me' — bumping someone, arriving late, interrupting. 没关系 méi guānxi is what you say back: 'it doesn't matter'. Both are frozen phrases, so do not try to take them apart yet. The 没 inside 没关系 is a negator you have not met — it belongs to level 4 and behaves quite differently from 不 — so for now leave it where it sits and learn the phrase whole. Notice too that 不 in 对不起 goes light and toneless.

谢谢。不客气。

The second pair. 谢谢 you already have; the reply is 不客气 bú kèqi, literally 'don't be polite', meaning 'you're welcome, don't mention it'. 客气 kèqi is the politeness people stand on ceremony with, and 不客气 waves it away. Because 客 falls, the 不 rises: bú kèqi, never bù kèqi.

欢迎！请坐。请问。

欢迎 huānyíng greets someone arriving — at a door, a shop, a classroom — and doubling it, 欢迎欢迎, sounds warmer still. Then comes the frame you met in 请问: 请 plus a verb is a genuine request, not decoration. 请坐 'please sit down', 请问 'may I ask'. It is short, it is polite, and it is the normal way to tell anyone to do anything. To leave, use 走 zǒu 'to go, to leave': 我们走 is the everyday 'let's go'.

对不起。没关系。谢谢。不客气。欢迎！请坐。

Say the two pairs aloud one after the other, both halves each time, until the answer arrives without thinking — that is the whole point of a formula. Then act out a small arrival: welcome someone with 欢迎，欢迎, seat them with 请坐, thank them with 谢谢, and when it is time to leave say 好，我们走.