Do you sign a lease before or after the application?
Assuming you mean leasing a house or apartment, you will fill out an application, and other people may fill out applications, too. You may need to pay an application fee as well. The landlord presumably will review all of the applications, do whatever credit and background checks that he requires, and decide who he will rent to. Then, if you are chose, you will sign a lease and pay the up-front money, and schedule your move-in date.