Most S/U-Bahn trains, buses and trams run every 5-15 minutes during the day.
Tickets can be used on all BVG, S-Bahn and local RE train services. Vending machines have instructions in English and accept coins, often bank notes and cards too. Berlin's AB travel zone contains nearly everything; you'll only need an ABC-ticket for Potsdam and Schoenefeld airport.
Before boarding the S- or U-Bahn, always validate your ticket by punching it in the yellow or red machines near the end of the platforms. On buses and trams, the machines are on board. Public transport uses the honour system, and there are regular checks by uniformed and plainclothes inspectors. If you are caught without a valid ticket you'll be fined €40 on the spot.