Go to settings then wifi and click the wifi to on. Now every time you are connected to your wifi it'll be wifi data. If you go outside your wifi area which is usually 50 metres then it'll go back to 3G. To know which it's using. On your phone to the left of your time is either a 3G symbol or a 3 bar fan symbol. This symbol shows you're using wifi and not the phones data 3G
*note. Once you've set your wifi to on you will have to link it to your router with the password set with the router. This is a once only. And once done will be forever connected as long as your are in it's area*