In addition to oaxacamatt's excellent advice, you can use Excel or create a Google spreadsheet to track your expenses over the course of a month, or even a week

In addition to oaxacamatt's excellent advice, you can use Excel or create a Google spreadsheet to track your expenses over the course of a month, or even a week; make budget categories, write everything down, and then start by using a week's or month's (or whatever period's) worth of data to formulate a target budget. Then create a budget-vs-actuals version of your spreadsheet. When you have a periods worth of data, see if you need to adjust your budget (hopefully downwards, but you don't want to be shooting for some unrealistic numbers, so do adjust categories upwards if you need to do so to be realistic), and then continue to compare periods of actuals-vs-budget. I also find it motivating to keep a list of "Yay!" and "Boo!" items, completely informally/separately from my spreadsheet. It's just a thing where I can write down things like, "Yay! Brought my own coffee to work instead of buying out" or other such little victories. The Yays can be more motivating to me than the Boos, but it might help to track both to see where you think you're breaking your own budget. And this can all be adapted very easily to a work budget, startup or even an established business.