The best way to learn proper English is to read news report, and watch news on TV. Watching TV shows is a great way to learn casual English, slang words, understand culture reference and humor. If you have already watched these shows then you may recall the words used in the following dialogs.

He could be the first CUSTOMER.
Breaking Bad Season 4, Episode 5

...just to complain
about their CUSTOMER service.
The Big Bang Theory Season 3, Episode 17

Yeah, you're my first CUSTOMER today.
The Big Bang Theory Season 6, Episode 1

Every CUSTOMER who
walked into that butcher shop...
The Big Bang Theory Season 3, Episode 13

Speaking of CUSTOMER,
it's not too late to have
a grand opening.
Breaking Bad Season 4, Episode 7