he big news in taxes this year is for married couples who work. There's a new deduction to reduce your income-tax bill. If you have young children at home, there's a bigger tax credit to help cover child-care expenses.

All taxpayers with earnings can save themselves money by starting an Individual Retirement Account. But two-earner couples get the best of it. They can each put aside up to $2,000 for a total tax deferral of $4,000.

All taxpayers also benefit from the cut in tax rates for 1982. But the biggest benefits go to those in the higher brackets, which tend to include many two-income families.

All these things together will save taxpayers anywhere from a few dollars to a few thousand dollars, depending on their income and circumstances.

But at any income level, a two-earner couple can do better than a one-earner couple with the same deductions, and both will do better than the single taxpayer.

The big changes this year:

(1) The Tax Deduction for Working Couples: Congress passed this special deduction to alleviate the so-called marriage penalty that afflicts spouses who both hold jobs. Working couples pay more tax on their combined incomes than they would if they filed as singles--the reason being that their combined earnings push them into a higher tax bracket.

The new deduction does not eliminate the marriage penalty, but it reduces it. To earn this reduction, you will have to struggle with a brand new tax computation guaranteed to drive you nuts.

Here's how you figure the working-couple deduction, roughly speaking:

Add up how much each spouse earned last year from wages, salaries, tips, commissions and self-employment income. Deduct each spouse's qualified business expense and Keogh or IRA contributions. Then compare the two incomes. You may deduct 5 percent of the lower income from your combined gross income (up to a maximum deduction of $1,500). This exercise reduces your income and your tax bill.

The working-couple deduction does the most for spouses whose incomes are equal (or where the lower-earning spouse earns at least $30,000). Take, for example, two couples earning $60,000 between them. The first couple earns $30,000 each. They can exclude $1,500 from their gross income and pay taxes on a net of $58,500. But the other couple splits the income differently. One earns $50,000, the other earns $10,000. They can exclude only $500 from income. And will have to pay taxes on $59,500. So this new tax break helps some working couples more than others.

(2) The Child-Care Credit: The most important thing to say about this valuable credit is that many working couples lose it accidently. They file the short tax return 1040a, which has no place to claim the credit. Not noticing it on the tax form, they forget it. To get the child-care credit--which directly reduces your income-tax bill--you must file the long form, 1040, even if you take the standard deduction.

The credit has been increased this year, with the biggest savings going to people with the lowest incomes. Working couples earning $10,000 or less get a credit worth up to $720 for one child and $1,440 for two children or more. The credit declines as incomes rise. It tops out for couples earning $28,000 or more, who may claim up to $480 for one child and $960 for two or more. Last year, all working couples got no more than $400 for one child and $800 for two.

As long as both parents work, or one works and one goes to school full-time, you can use this credit to offset the care of a child under 15 or an older disabled child or other dependent. The credit also is available to a single parent who works or is a full-time student, while taking care of a child; or to a working person with a disabled dependent spouse who cannot take care of himself. 'At any income level, a two-earner couple can do better than a one-earner couple with the same deductions, and both will do better than the single taxpayer.'

(3) The Individual Retirement Account: This is the first tax year that all working people can invest up to $2,000 in an IRA ($2,250 if you have a nonworking spouse) and deduct that money on your income-tax return. You may start the IRA as late as the due date of your tax return this year and still deduct your investment against last year's income.