Is your Wi-Fi network at home password-protected? If not, it should be. You might not care if your neighbors use your Wi-Fi connection to surf the Web, but someone with more sinister motives could take advantage of your generosity (and lack of protection) to gain access to data stored on your home PCs.
The easiest way to guard against Wi-Fi interlopers is to encrypt your Wi-Fi network. Afterward you'll have to enter a password whenever you connect to your Wi-Fi network, but that's a small price to pay for improved security. Most Wi-Fi routers support WEP, WPA, and WPA2 encryption standards. Be sure to use either the WPA or WPA2 encryption settings, which provide a much higher level of security than WEP encryption.
Another safeguard is to set your router not to broadcast the SSID (your network's name). With SSID broadcasting disabled, your wireless network won't be visible to computers nearby, and only people who specifically know your network's name will be able to find it.
The procedure for locking down your Wi-Fi will vary depending on your router's model and manufacturer. Check the router's documentation for instructions.
Note that using a public Wi-Fi network safely entails adopting a different set of security procedures.