Begin by listing your income for the month. This should include any paychecks you receive, as well as income from other sources like child support or investments. If you have a business, you should include the amount that you pay yourself out of the business each month.
Finally, do it again. After you have completed your first month of budgeting, it will be easier to plan for the next month. At the end of the month look at how you spent and make adjustments for categories in which you spent more than you planned, and cut back on the categories that had additional funds in them.