NOW COMES THE moment, President Carter says, when this country gets serious about energy conservation. That's splendid. It's crucial to remember that of all those intricate energy options that people keep talking about - from the breeder reactor to solar energy - conservation is the only one that will make any difference by next winter. Or, for that matter, within Mr. Carter's four-year term of office. It you don't like the way things are going this winter, there's only one pratical remedy.

The next energy shortage, incidentally, might not be as far away as next winter. If the country were to continue to coast along, postponing the tough choices and hoping for the best, there is a better-than-even chance of gasoline shortages next summer. The only real issue is how to cut down.

Voluntary cooperation will help. Regulation can also help through the kind of rules that limit the heating and cooling of public buildings, for example. But sustained and significant conservation, on the scale now necessary, is going to mean massive read-justments by people in their private lives and businesses. It's not going to be fun. The best way to achieve it, and probably the only way, is through a careful program of increasingly stiff energy taxes.

Suppose that the country were to decide on a federal gasoline tax that rose a nicket every six months for the next five years. It would take the price of gasoline up only a little this summer, but well over $1 a gallon by 1982.

The virtue of a tax rising over the years is that it does not fatally jolt the economy with one massive unexpected blow. But it would warn everybody that the cost of driving is going to rise sharply. As you brought your next car, looked for your next home and planned your next vacation, you would know what to expect.

But what about the impact of this tax, and its effect on the poor? It would be necessary to pour that gasoline tax money back into the economy as fast as it was collected. Each penny of tax on a gallon of gasoline raises $1 billion a year, and tax increases of the magnitude could throw the whole economy into recession. As a solution, our own first choice would be to balance increases in the gasoline tax with comparable cuts in the highly regressive Social Security payroll tax. If that tax were cut from the bottom up, it would provide equal benefits to every wage earner in the country. Those who kept driving a lot would come out behind, and those who cut down their driving would come out ahead.

It's important to discriminate, too, among the various uses of oil. It's better to put most of the next tax load onto gasoline, than to put it on home heating oil. Rapid increases in heating fuel costs are excessively harsh on the very poor and people on fixed incomes. The central defect of the Ford administration's energy tax plan, two years ago, was its attempt to put the tax on crude oil, raising all oil product prices indiscriminately.

A reader called, not long ago, to object that this kind of talk about gasoline taxes was a threat to his freedom of movement. Perhaps that's true. But transportation has never been free. It's not as through as gasoline prices were otherwise going to hold still. They are going to rise, whatever Mr. Carter and Congress do. In the past, Americans have left the decisions - and the revenues - with foreign oil producers. As the past months and years should have taught us, the costs of running out are far higher than the costs of saving.