Buy tickets from the Amsterdam Tourist Information Offices (called "VVV" offices) for Bus 58 from Schiphol direct to the gardens. The buses leave every 15 minutes from the second island outside the arrivals-level entrance at the airport. See the Keukenhof website for more information.
Note: there are tour outfits that charter buses for groups to go to the Keukenhof. I prefer to do things like this on my own, using public transportation. Ask the VVV offices for tour group information.