I don't get how Mint determines your 'budget'. It has all kinds of weird numbers for me that I don't remember setting. Is that something they do automatically based on past spending in those areas, or is this something I can control?
Mint Budgets uses averaging based on past spending initially. You are able to control the value of the budget for each category though. If you are on the Budgets page, hover over the specific budget you want to adjust and click Edit Details. A dialog will come up that allows you to set the frequency, amount, and whether or not the value rolls over to the next period. You can also delete an existing budget from this dialog.
On the Budgets page, there is a Create Budget button you can use to add a new category budget.