After you complete your profile, you'll be asked to confirm your phone number. You should receive a text message with a six-digit numerical code. Check your text messages to find this code.
On the iPhone, you may also see the code appear above the keyboard – tapping it will enter the code automatically (so you don't need to check your text messages to retrieve it). Enter the code on the confirmation screen. If you do not receive the code, press the Send it Again button.