There are many ways to start a budget. Some people just dive right into a software or an add on on their bank website. Or they plug in numbers in an Excel sheet. I spend about this on rent this on utilities, etc, etc. I don't recommend this haphazard way of budgeting. For one, it's not giving you a realistic picture.
And for two, it will leave you with more work in the long run when it comes time to update and you realize nothing was correct in the first place. That being said, I recommend you set aside some time in your schedule to do this right. To prepare yourself for this first step, I recommend you get everything you need beforehand. Don't worry though, it's simpler than you think. All you need to start your budget is to number one. Make or have copies of your bank credit card and loan statement.
As well as copies of your last three paychecks, and number to open up a fresh excel sheet on your computer, or pull out a sheet of paper and a pen. And number three, lastly, open up the calculator app on your phone. Or if you actually have a calculator dusted off and set it on your desk. Once you're prepared, step two will be a breeze.