Once landing in Schipol International, catch the direct railway to the Amsterdam Central Station. It is the fastest and most convenient way to enter into the city centre. Once in the city, the preferred mode of transit is to simple walk. Many major sites are in and around the city centre. Attractions further away are easily accessible by public transport. Make sure to buy a metro card before exiting the Central Station. You will need euros.
If your time in Amsterdam is short, for instance, an afternoon layover. With the metro being above ground, I highly recommend taking this form of transit, especially if your time in Amsterdam is limited. Just riding around the city for an hour will give you a great feel for the neighborhoods. Here are a few photos taken from riding around on the metro: