To add an Icon as a menu item:
Step 1: Go to Appearance > Menus.
Step 2: Click on the Links section from the left vertical menu.
Step 3: In the Link Text box, type the menu element’s name after or followed by <i class="fa fa-credit-card"></i> to add an icon before or after the text. (Where fa-credit-card is the item code).
For the complete list of icon codes go to http://fontawesome.io/icons/