The U.S.-Soviet arms talks scheduled for Geneva next month resulted from a Soviet initiative that was accepted almost intact by the United States, according to administration sources.

One of the complications facing President Reagan and his senior policy-makers in their highly secret discussions concerning the Jan. 7-8 meeting is that they are far from certain just what Moscow has in mind.

The nature and scope of next month's talks were defined in the first instance by a message from President Konstantin U. Chernenko that came as a complete surprise when delivered here Nov. 17. The administration quickly accepted, responding in language nearly the same as that used by Chernenko and inserting only the time and place for the meeting of Secretary of State George P. Shultz and Foreign Minister Andrei A. Gromyko.

The way all this was done makes it more complicated than usual to plan U.S. strategy.

Public statements by Kremlin officials since the Shultz-Gromyko meeting was announced suggest that the Soviet position is likely to be similar to that expressed by Chernenko in statements to The Washington Post and NBC News.

These points were essentially repeated in a speech Nov. 21 by Politburo member V.V. Shcherbitskiy. He said Chernenko had "reaffirmed the Soviet Union's readiness to embark on talks aimed at elaborating agreements on the prevention of the militarization of space, on a quantitative and qualitative nuclear arms freeze and on the complete and total prohibition of nuclear weapons tests."

Four days later, a Radio Moscow broadcast said the Soviet idea is "to conclude an agreement to prevent militarization of outer space, to freeze nuclear armaments and to fully ban nuclear weapon tests."

Deliberations on how to respond, according to guarded hints from officials who have been required to sign an unusual oath of secrecy, center on these questions: How negotiations on diverse problems -- defensive weapons in space, offensive strategic arms and intermediate-range arms -- can be structured. The Soviets persistently inquire which subject should come first and, in effect, receive highest priority. The Soviet priority, as suggested by public statements, is on stopping the U.S. "Star Wars" program and other military efforts in outer space. The U.S. priority is on reducing the level of offensive nuclear weapons. What "interim restraints" will be offered by the United States on its own military programs during the course of the negotiations, which officials believe may last for years. Whether a "special envoy" is to be appointed to take the lead in negotiations, and whether some way will be found to resolve disputes within the administration.

The most explicit disclosure of the origin of next month's meeting came from Reagan himself Tuesday, in a generally overlooked sentence, almost an aside, in an interview with The Washington Times.

Asked about the arms negotiations, Reagan said he believed the Soviets were "kind of stalling" until after the U.S. presidential election. Then he added, "They've made a proposal and we've said fine."

Chernenko's surprise Nov. 17 message, according to informed sources, did not mention the U.S. concept of "umbrella talks" covering at least six arms control issues, which had been announced by Reagan before the U.N. General Assembly Sept. 24 and presented in private to Gromyko at the White House Sept. 28.

Instead, Chernenko straightforwardly proposed a Shultz-Gromyko meeting to decide the subject and objectives of new negotations covering militarization of space, strategic nuclear armaments and medium-range weapons in Europe.

None of this is cause for great optimism about an early breakthrough. One seasoned U.S. diplomatic observer said that, based on the comments to date, Gromyko can be expected to arrive in Geneva with a very tough position coupled with a willingness to negotiate.

Wary U.S. officials told visiting West German Chancellor Helmut Kohl during a White House meeting Friday that the Shultz-Gromyko meeting is likely to be only a first step in the direction of opening negotiations, and that follow-up political meetings may be necessary before formal bargaining begins.

According to the account of a participant, Reagan and Kohl discussed the high level of public expectation that has been aroused and agreed that patience would be needed and that the expectations should be lowered.

Another U.S. concern is that the Soviets may use the Geneva talks as a way to revive the political ferment in western Europe against continuing deployment of U.S. medium-range missiles.

Last week it was reported that the Belgian government might decide not to go ahead with the deployment of U.S. ground-launched cruise missiles beginning next March. To head off such a move, Assistant Secretary of State Richard R. Burt was poised to make a secret trip to Brussels armed with a letter from Reagan.

But in the end Burt stayed home and the Belgians merely postponed a deployment decision until after the Shultz-Gromyko meeting and a Jan. 14 visit to Reagan by Belgian Prime Minister Wilfried Martens. U.S. officials said they were "satisfied" with this.