Does anyone know how to build a bar? How do you do it?

You can built the main carcass out of 2x4's and plywood then add a nice top and finishings (oak, tongue and groove, etc) on the exposed area.
On the bartender side it doesn't have to be pretty. Install shelves, small frig underneath for the drinks, glass holders, etc.

If you want to serve beer through a tap, there are a couple of things you will need..

1 x keg fitting
1 x cooler(undercounter)
1 x gas valve Co2 for lager or mixed gas for bitter
1 x tap and bracket

you also need pipes to connect the keg fitting to the cooler and piping from gas valve to keg fitting. Then last of all pipe from cooler to the tap.

it sounds a lot harder then it is... Trust me I work for a brewery