If you are new to London you will find that the city is divided into travel zones which will dictate the cost of your journey by underground (tube), rail or bus.
While you are looking for accommodation it is likely that you will have to view several properties in different areas in one day.
The best way to keep travel costs down is to buy a one day travel card, or better yet buy a standard adult Oyster card (until you are able to apply for a student Oyster card) from any tube station or location where you see the Oyster card logo. These allow you unlimited journeys by tube, Docklands Light Railway (DLR), bus and some National Rail services within greater London throughout your selected zones after 9.30am Mondays - Fridays; and all day on Saturdays, Sundays and public holidays.
For more details please see the Transport for London website. This is especially useful for information about public transport in London, with details about all bus and tube journeys.