I would spend a weekend or two checking out the places you want to work at and
see what their weekend nights are like. Then go back during the week to the
place(s) that you liked and sit down at the bar and have a conversation with
the bartender.
Generally speaking, you don't need any experience to be a barback. And the
experience you gain in that position will go far towards whether they allow
you to work the bar. Remember that, in some establishments, servers get first
crack at any open bartending positions.
Good luck getting behind the bar.
You can learn everything you need to know about bartending for Free at the
link below ;)
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http://www.Bars-and-Bartending.com