At its most basic, a budget is just a plan of what to spend or what to save so you know what extras you can or cannot afford.
Everyone knows it's a good idea but too few people seem to bother, yet it doesn't have to be difficult.
It really is important to know how much money is coming in and where that money is going out. It helps you prioritise your spending to cover what's most important and helps to reduce the uncertainty and stress of money troubles.
The ASIC Money Smart budget planner, available here, is an easy to use tool to summarise all your income and expense categories and see where you might save, or spend more. You can save your budget online or use it offline as a spreadsheet.
If possible, you should print out your budget and see how you are tracking against it each week or month. That will help you identify small issues before they become big problems.