You find these machines at most subway stations. Information is available in English. You can put in both coins and bills. Be careful with the bills. Some machines repeatedly spit them out again. The machines do refund your money. It is best to buy either a twenty-four-hour ticket, which lets you use any BVG (Berlin public transport company) vehicle as often as you want. There are also tickets for 5 rides, family tickets, tourist tickets and monthly tickets. The latter are expensive (ca. DM 100).