You will find this setting in your Wi-Fi router settings (in the settings on your iPhone or iPad). If you don't see this option in your settings, you should contact your ISP for more information about how best to set up your Wi-Fi encryption. Ideally, you want to use WPA2 - AES encryption or WPA2 - TKIP encryption (most modern routers support one or both). These two types of encryption are considered highly secure, so they'll provide the best possible protection for your home network.