WHAT IS IT that Americans should want from the Soviet Union, anyway? How should they go about getting it? It is all very well, if you don't get ponderous, to watch Ronald Reagan and Mikhail Gorbachev exchange New Year's greetings, but what about the morning after?

Any reasonable answer has to be something more -- that is, something less -- than a political wish list. For instance, Americans wish that the Soviet Union would stop amassing new weapons, manipulating weaker nations and oppressing its citizens. The Kremlin's self-license is unquestionably a leading cause of world tension and human unhappiness. Yet to expect the Soviet Union soon to change its totalitarian nature is far-fetched, and to make such a change the unbendable condition of limited accommodation is self-defeatin.

Intellectually speaking, it is easy to see what the Soviets must do if even modest accommodation is to be accomplished: they must show restraint in its various forms in foreign policy and at home. Compared with that, the task ahead for American policy is considerably more complex. President Reagan remains a good distance from working out the necessary new balance between his deeply rooted tendency to confront Soviet power and his relatively recent inclination to experiment with better relations. He must do this in his personal thinking, in his management of American politics and in his conduct of foreign policy. Which of these hurdles is higher? In theory, there exists a suitable destination for him: Richard Nixon's "hard-headed d,etente," a policy combining toughness and self-interested reasonableness, a readiness to confront the Kremlin where necessary and cooperate with it where possible, to assert the American interest but to recognize some common Soviet-American interests too. The elements of this policy are difficult to match and to sustain. No president, whether he started from left, right or center, has yet done it successfully.

Mr. Reagan has made his own path more difficult by embracing as an operational project the goal of achieving a complete anti-missile defense. He says research will let Americans determine whether his Strategic Defense Initiative is feasible, effective and a good value. The Soviets say they couldn't care less. They call SDI a shield behind which the United States could threaten attack, and they claim it forecloses the deep cuts in offensive arms otherwise open.

The burden is on Mr. Gorbachev to act in a manner vis-dup and their foreign adventures -- especially in Afghanistan -- that will make it possible for the Americans to do the limited business both he and the president set forth. The burden is on Mr. Reagan to decide whether some measure of accommodation is possible and desirable and to ensure that the SDI project helps, not hurts, the quest for security. This is the morning after.