Purchase a WiFi dongle and make sure that it is compatible with the Pi using this list of compatible products.
Connect the adapter into one of the Pi's USB slots. Boot up the Pi, login to the pi and go to the Pi's GUI. Navigate to the Pi's start menu on the top left corner of the GUI. Select the application settings and then click on the WiFi configuration program. With your internet ssid and password handy you should first press the scan button then select and double click your network as it comes up. You will be prompted to enter in your password, after you have done this you can click connect. The WiFi application should then set up the connection and tell you in the status bar that the internet is connected.
If this doesn't work or your ssid isn't coming up your WiFi adapter possibly has some compatibility issues with the Pi. To solve these problems it is a good idea to look up your adapter on the Raspberry Pi's list of compatible accessories and then follow the links to the find the available drivers for your adapter.