If you are staying in Tokyo for a while, and wish to use the public transportation system frequently, you might as well get a transportation card.
There are two companies that provide transportation cards. Suica and PASMO. Both work the same way, and allow access to almost all trains/metros/buses in Tokyo. The biggest advantage of this card is that you don't need to buy a ticket from the vending machine every time you travel. Sometimes, there can be a huge line, and getting tickets for transfers can be time consuming.
The PASMO card can be purchased very easily from most of the metro stations in Tokyo. These cards require a 500 yen deposit, and can be recharged for up to 10,000 yen. Over 100 transportation facilities accept the PASMO card, and it is very simple to commute around the gigantic city of Tokyo with this little card.
The PASMO card can also be used for electronic money payments. For example, if you want to purchase a bottle of soda from a vending machine that accepts the PASMO card, you can simply scan your card and get your drink. Most of the soda vending machines in metro stops accept the PASMO card.
To get these cards you need to look for the PASMO logo in a vending machine. After you click the 'PASMO' option, and move on to the next screen, you need to select the first option that allows you to purchase a new PASMO card. The other two options are for recharging and deposit refund.
(NOTE: At anytime of your purchase, you can opt for English instructions.)
Finally you select the 'general user' option and then select the initial credit value in the card. Once you no longer need the card, you can always get the deposit back after you have returned the card to the vending machine.
As a tourist, having change at all times may be difficult, and hence it is highly recommended to have a transportation card. Not only does it save time, but it is very convenient to have in emergency situations.