First things, first. Here are four idioms and natural English expressions which using the word first.
A member of a group who has more power (or is more important) than the rest, even though officially the members are all on the same level. One person is higher than the rest when they should all be the same.
"The Pope is first among equals in the Roman Catholic Church."
It's important to look and act your best when you meet someone for the first time because people will remember the way you appear when you first meet them. You never get a second chance to make a first impression!
"Because Tim knew that first impressions are the most lasting, he wore his best suit to the job interview."
This is Australian English. This phrase is used when the first people to do something get something first or will have an advantage. In British English the idiom 'First come, first served' is used instead.
"The radio station are giving away five concert tickets, so it's first in, best dressed."
In horse racing the winner is the first horse to pass the winning post. First past the post means that someone was the winner or the first to do something.
"The Americans were the first past the post in the race to the moon. Neil Armstrong walked on the moon in 1969."