You need documentation showing your income and the income of your spouse. If you and your spouse are salaried employees, you will need a copy of the most recent pay stubs plus your most resent Income Tax Return.
Determining income becomes more difficult if your spouse is self-employed. In such a case, copies of bank account statements and financial business statements will give a clear picture of income. It is always a good idea to make copies of such statements before separating and filing for divorce.
You may be able to get an idea of how much your spouse actually makes but, it can be almost impossible to determine true income when a spouse is self-employed. Gather what information you can and then your attorney can help get the rest through the discovery process.