A budget is a plan, a financial plan. It sets out your estimated income and expenses for a period of time. The expenses are broken down into categories such as rent, groceries, phone and so forth. Your income is broken down into categories such as salary, dividends and bank interest.
One does not simply dream up numbers. The figures in a budget must be the estimated costs of planned activities. The costs are what you estimate to be the best deal available, in other words the deal you can get after shopping around. To prepare a budget you must have a clear idea of what you are going to do and how much it costs.
You can not estimate your grocery costs for a year if you do know how much you are spending each week. Write down all of your expenses for several weeks so you have accurate information about all of your usual weekly expenses. Get estimates of your bills and then you can prepare an annual budget with some measure of confidence.
The costs are the consequence of activities. This is an important point because if you wish to change your costs you must change your activities. A budget all by itself will not save you money. It will help save you money by telling you when you heading off track.
To save more money you must change your behaviours.