After contracting to buy a home, one applies for a mortgage (unless you pay cash of course). It's very important to have your documents organized before applying, as the application interview process will go very smoothly. Being prepared also reduces the likelihood of documents being requested before the closing.
Applying for a mortgage used to be intimidating. But today there is great competition in the lending market, and automation of credit scoring and reporting has simplified the process. At the application interview, you will be asked a lot of questions and be required to sign many documents (receipt of truth in lending statement, approval document to access your credit report, etc).
Below is an example list of documents you should prepare before a mortgage application interview: