Though I've never been great at following through on it......a great way to start is to keep ALL receipts of ALL expenses for one month. Don't change ANYTHING that first month so that you can see exactly what you're actually spending. Then you can decide on where to cut back.
The biggest part of keeping up with the budget is actually keeping up with keeping TRACK of what you spend (that's the part I'm bad at!)
Like I said, I've never gotten much further than the keeping track part, haha! But that should be a good place to get started