Depends on where you live. Small town where the clientele is mostly fat guys in greasy ball caps and skanky women who fall off their bar stools? You need to know somebody. And then you'll only make about $10/hour on the best nights, because those people tip worse than teenagers on an allowance.
But in a place where it's lucrative, like a city or colleg town, you'll most likely start by being a server and/or bar back for at least a year. And even then it helps to know someone or be a really good looking woman. I've seen guys spend years trying to catch on as bartenders and then only get the slowest shifts of the week. While hot little 21 year-olds who don't know the difference between rum and whiskey get hired just to attract the sausage crowd.