If you want a quick high level answer, just go into your router settings (as specified in your router manual) and make sure you enable WPA authentication. Then pick a good password (random assortment of 14-20 characters, symbols, and numbers). I would also change the admin login password for the router to a separate, equally complex password. The most important thing here is to choose WPA or WPA2; most routers default to WEP. DO NOT EVER USE WEP. I personally have cracked WEP routers in less than 3 minutes; the encryption is a total joke. There are other, more complex things that you can do to secure your router and home network but for the average person this is the basic solution.