Turn on your WiFi from either your Settings or from the drop-down (or up) taskbar on your device.
You will have the choice to join a number of networks in your near vacinity, choose your network SSID (name) Ex: Johns WiFi.
If you have a password set to it, there will be a window prompting you to enter a password; enter it if you have it handy.
Depending on the device you may find the WiFi setting in certain areas in your device, most likely the WiFi setup will be the first in the list of settings.