I'm a fan of MS Excel _ my apologies for those who have issues with Microsoft _ because I find its powerful calculations are easy to manipulate to help me better understand my finances. It doesn't really matter though which spreadsheeting package you use so long as you are able to calculate your amounts effectively. There are commercial budgeting worksheets available but if you want to create your own its a simple process.
The second thing we need to do is to find out how much money you actually spend each month again include everything even those items that your spouse does not know about like that Latte down at the coffee stand, that you get on a daily basis. It helps to break this down a little. Fixed nonnegotiable expenses, things we actually cannot do without, we all need food lodging medical care basic clothing insurance property taxes a car may be necessary where you live, you get the idea.