Secretary of State George P. Shultz yesterday called the Lebanese-Israeli peace agreement "the essential first step" toward getting all foreign forces out of Lebanon, and he predicted that the plan will succeed because all parties, including Syria, "will realize that the risks if the withdrawal process fails are greater than the risks of completing it."

President Reagan, leaving the White House for a weekend at Camp David, was asked by reporters if he thought "Syria would come around." He replied, "I'm going to continue to hope that they will. We're still working on that."

Shultz, who returned Wednesday after a 2 1/2-week Middle East trip in which he negotiated the agreement for withdrawal of Israeli forces, spoke to the Business Council in Hot Springs, Va. The text of his remarks was released by the State Department last night.

"We have only made a beginning, but it is an important beginning," Shultz said in reference to the fact that implementation of the accord depends on the agreement of Syria and the Palestine Liberation Organization to pull their forces out of Lebanon in conjunction with Israel's withdrawal.

At a meeting with Shultz in Damascus last Saturday, Syrian President Hafez Assad expressed strong hostility toward the agreement, and Syria since has continued to attack the accord for allegedly giving Israel unacceptable gains in Lebanon.

However, Shultz reiterated that the United States does not regard the Syrian position as a rejection of the agreement. He stressed U.S. determination to work at persuading Assad of the agreement's importance to Middle East peace, and he hinted that the effort will involve American attempts to achieve a better relationship with Syria.

"The Syrian government treated us with great courtesy," he said of the four-hour meeting with Assad. "I know that Syria--like Lebanon--will make its own sovereign decision on an issue so important to it. Both Syria and the United States regard a renewal of contacts and improved relations as in their mutual interest.

"And all parties will realize, I am sure, that the risks if the withdrawal process fails are greater than the risks of completing it," Shultz added.

The Reagan administration hopes that it can marshal support from moderate Arab governments like Saudi Arabia, which gives Syria substantial financial aid, to persuade Assad that it is in the larger interest of the Arab world to help restore Lebanon's independence.

In addition, the administration is known to be considering the possibility of taking a direct hand in negotiations between Syria and Lebanon, similar to the effort it made between Israel and Lebanon. If Syria reacts positively to that idea, the U.S. mediating effort will be entrusted to Reagan's special Middle East envoy, Philip C. Habib.

"Syria is a proud country, and it has legitimate security concerns with respect to Lebanon," Shultz said. "I was able to tell President Assad that the purpose of my mission was to start the process of restoring Lebanon's sovereignty over all its territory, and withdrawing all external forces, which would enhance the security and well-being of all of Lebanon's neighbors."

Shultz also said his trip had convinced him that Reagan's larger Mideast peace initiative was still alive despite the failure of Jordan's King Hussein to win the PLO's permission to represent the Palestinian cause in expanded peace talks with Israel.

"Our friends in the Arab world are still supportive of President Reagan's initiative and are actively working to put together a positive Arab response," he said.

At the National Press Club yesterday, former Israeli defense minister Ariel Sharon said that the United States should immediately warn the Soviet Union against encouraging trouble in Lebanon.

He pointed to Soviet aid in rebuilding the Syrian armed forces and said that while he does not believe Moscow wants a new war, it is encouraging shellings and ambushes to force Israel and U.S. Marines in the multinational force out of Lebanon.