If you need some help getting started, start here. 

Equitable Distribution (Division of Assets and Debts)

Identify every asset and liability between you and your spouse. Typical assets include real estate and cooperative apartments, brokerage accounts, savings accounts, checking accounts, retirement accounts, cars and other items of significant value such as collections and jewelry. Typical liabilities include mortgages, credit cards and loans. If possible, gather the statements for these items at the time you got married and for the last three years. If you cannot gather the statements for the last three years, grab the statements at about the time the summons in your divorce action was filed (assuming a summons has already been filed) and the most recent statements. 

Support Obligations

To calculate support obligations, you want income information. For the last three years, try to obtain copies of the tax returns including support documentation such as 1099 and W2 statements. You also want earnings statements for the last three pay periods (most people are paid weekly or bi-weekly). If someone is paid off of the books, you'll also want bank statements to track deposits.

Parenting/Custody

Start documenting the current parenting schedule. If you are the non-custodial parent, know your child's providers such as babysitter, teachers, guidance counselor, therapist and doctors. Know your child's extracurricular activities and instructors. Know how your child is doing in school and help with the homework. If you are not involved in your child's life - get involved! If you happen to be dating someone, do not introduce your new boyfriend or girlfriend to your child! 

Statement of Net Worth/Financial Disclosure Affidavit/Income and Expense Statements

In a divorce action, you will need to complete a statement of net worth. In a family court action for support, you will need to complete a financial disclosure affidavit or income and expense statement. You may want to get these documents started sooner rather than later. Even if you don't start filling in the paperwork, start paying attention where you spend you money. If you just went to the grocery story, how much did you spend? Is that more or less than you normally spend? How much do you shell out for school and work lunches? What is the cost of a trip to the hair salon?

Even if you can't obtain this information, don't let this stop you. In this day and age its more difficult to obtain certain documents as the accounts are accessed online only. This means that you will need to work with your attorney to obtain this information. Having the information at the start makes the process easier, not impossible. 

By failing to prepare, you are preparing to fail.
—Benjamin Franklin