Secretary of State George Shultz admits it will be hard to end the fighting between Afghanistan's rebels and the Soviet-backed government but predicts the withdrawal of Soviet troops will bring some stability to the war-torn nation.

"I . . . expect that the Soviet Union will withdraw fully from Afghanistan, and then the people of Afghanistan have got to work things out," Shultz said. "That's their right and their problem."The secretary met with reporters after President Reagan announced on Monday that he was dispatching Shultz to Geneva to sign the troop-withdrawal accords negotiated last week between Afghanistan and Pakistan.

Reagan lauded the U.S.-backed rebels, who have battled the Kabul regime since the 1979 Soviet invasion.

"This development would not have been possible had it not been for the valiant struggle of the Afghan people to rid their country of foreign occupation," Reagan said. "We take great pride in having assisted the Afghan people in this triumph, and they can count on our continued support."

Asked about charges from conservatives that the agreement is a sellout of the rebels, Reagan replied, "We are not."

Under the agreement, the United States and Soviet Union will guarantee a plan dictating the removal of the Red Army's 115,000 troops from Afghanistan.

The plan has been worked out under the auspices of the United Nations and is expected to be signed on Thursday.

The pullout is set to begin May 15, only two weeks before Reagan is scheduled to fly to Moscow for his fourth summit session with Soviet leader Mikhail S. Gorbachev.

The rebels, or mujahedeen, were not part of the talks. They have rejected the agreement and pledged to continue fighting to topple the Kabul government.

Yet under a compromise with Moscow that is not a formal part of the accords, the United States will continue sending weapons to the reb-els, while the Kremlin continues providing arms to the Marxist government.

Asked if U.S. pledges to continue supporting the rebels contradict the formal agreement, Shultz replied only, "I'm not going to step into that hole."

He said the United States would continue to aid the rebels "as needed."

"We have stood by the Afghan people during this 81/2 years, and we're prepared to stand by them in the coming months to help Afghans return home and begin rebuilding their lives and their country," he added.