how you connect to Wi-fi will depend on the phone you are using. Here is how you do it for for most phones:
1) Go on Apps, Settings and turn on Wi-Fi
2) Choose the network you want to use and tap on it
3) Enter the network password
If you are using your home's Wi-Fi, the network name and password is usually on the the modem or on a paper when they set it up for you. If you lost that information, you will need to contact your internet provider to get that information.
If you have access to Wi-Fi, it's a good idea to turn off the mobile data to make sure your Wi-Fi connection doesn't accidentally drop and uses the data on your plan.
Hope this helps!