The expression first come, first served is similar to "the early bird gets the worm."
If the new bakery in town offers free cookies first come, first served, you better get in line right now before they run out. There's no making a reservation, no buttering up the owner: the first people there get the cookies. Businesses use this tactic to attract customers. You can add hyphens if the phrase is before a noun, as in "tickets will be available on a first-come, first-served basis." Though you'll often see a comma between the two halves of this expression, sometimes it is used without one.
Below are some examples of the phrase spelled correctly. Both refer to limited quantities. Check it out:
The Clippers sell $10 tickets at Staples Center on game day — first come, first served, with 50 to 200 tickets available for each game. (Los Angeles Times)
There are six camping sites available on a first-come, first-served basis. (Washington Times)
Sometimes the "d" is left off of first come, first served and the phrase turns into first come, first serve — but that makes it sound like if you get there first you have to do the serving instead of being served. Served needs that "d." It's the same kind of "d" that falls off of "iced coffee" and "iced tea." Put it back!
Even well-known publications sometimes get this phrase wrong:
A limited number of virtual "tickets" to each film are granted – usually free – on a first come, first serve basis. (The Guardian)
The sneakers will be available first come, first serve until they sell out. (Seattle Times)
If there's something you want on a first-come, first-served basis, then don't hesitate! Get served — with an -ed