In bars and restaurant you will be expected to tip the waitress or the bartender, as in most of the western world. The customary tip is 12-15% of the account (service is not included by law). It is given to the server after getting your change back, or just left on the table (or in the paying case if given). If you pay with credit card, in some places you might ask to add it to the account’s charge.
If you got a horrible experience it is not a mandatory to tip, but if you got an extra service it would be nice to extra-tip the person – most of waitresses and bartenders earn a minimum wage and those tips are part of it.