Yokohama does not have its own airport. You can reach Yokohama from Tokyo's two main airports.
Some Narita Express trains from Narita Airport go through to Yokohama (1 1/2 hours). Limousine Buses run frequently from Narita to the Yokohama City Air Terminal (YCAT) (1 1/2 ~3 hours,). The cheapest access from Narita Airport is to take a Keisei train to Nippori, then change to a JR Keihin-Tohoku Line train for Yokohama (about 2 hr. and).
From Haneda Airport, take the Keikyu Line's Airport Express to Yokohama station in 30-35 minutes . Note that the Airport Express has several variants: If the train's destination is Shin-Zushi or Kanazawa-Bunko , then you can stay on the train for the entire trip. If the train goes to another destination, it's likely to continue on into Tokyo so you will need to change at Keikyu-Kamata station to the next main line limited express train on Platform 2.