The easiest way to make a field mandatory is to set the field to be required on the Page Layout.

You can accomplish this by doing the following: 
  1. Click your name in the top right corner and select Setup. 
  2. Go to App Setup -> Customize, click on the object (eg Contacts), then Page Layouts. 
  3. Click Edit next to the record type layout (eg Household Contact Layout). 
  4. Find the field you want to be required on the sample record page. 
  5. Hover over it and click the wrench icon. 
  6. Check Required. 
  7. Click Ok. 
  8. Click Save.