Even if math isn't your thing, it's important to figure out exactly what you're spending every month.
Add up all the money you have coming in -- generally, that's your paycheck. Then subtract what you regularly spend. If you're not disciplined enough to write down all your purchases, ditch the cash for a month and use a debit or credit card to get an electronic record of all your spending. Just keep in mind: people tend to spend more when swiping plastic.
If the result of your calculation is positive, consider putting the difference into savings. But if you're spending more than you're taking in, it's time to cut back.