You will probably need a developer kit from one of the companies that make chipsets like Atheros, Broadcom, or Intersil. You need to find a book on 802.11b (the most popular WLAN standard). What exactly do you want to use the device to do? Does it need to merely report and receive data? The application determines at what point in the OSI stack you need an API. You will essentially need to write a "driver" for your device that interfaces to the wireless card. You would probably plug a standard 802.11b (WiFi) PCMCIA card into the device and open a port to it using DHCP or static IP assignment.