There are a lot of things that need to be included in a budget, but the two basics are income and liabilities. The reason for a budget is to ensure that you have enough money coming in to pay all your liabilities (aka bills) and have enough to reinvest into your business. You will need to include all your expected income for the budget period and all expenditures such as rent, utilities, payroll, taxes, etc.
Once you have established your budget, it is important for you as a business owner to stick to it.