Tap the "WiFi" button, which will bring up a list of networks that are in range.

Tap a WiFi network, then press "Connect" in order to connect to it.

If you see a lock on the network, you may need a password in order for you to gain access to the network. Enter the password then press "Connect".

If you don't know the password for the network you are trying to connect to, try contacting the person who set up the network.