Or, 'how to behave yourself in a public place'.
First off, you've come in to an established setting, in which there is a bar. The owners and some other established staff members work here, and if you are relatively polite, they will get you a drink, or anything else we have on offer. (Please see Food, Drinks, and Rooms for more on any of those.) If you're a dick, they'll treat you accordingly. They are not your servants, nor is this your living room. Do not, under any circumstances, attempt to get behind the bar or serve your own drinks, without being a staff member. It will probably result in Bad Shit™. IC, there are very powerful wards around the bar and kitchen, which only staff and family members can pass.
If no staff are present, it is safe to assume that there are a pitcher of beer and a decanter of wine, along with an honour box, on the bar. Both are relatively cheap, with the beer at 7cp a glass and the wine for 1sp. You may serve yourself either, or get water from the wash-pumps in the public restrooms at the front of the buildng. There is also a near-infinite supply of beer-nuts in bowls along the bar. These are free. Enjoy them.
For more on potential IC consequences of IC actions, please see the 'World' section, under Info, where wergild, hospitality, and, in the future, other IC cultural concerns are discussed.