By next autumn the first small steps will likely have begun: the planting of slender-branched willow oak trees along Pennsylvania Avenue, the widening of sidewalks, some changes in the alignment of Washington's ceremonial "main street."

Then, if all goest according to plan, a major stride will be taken: the restoration of the historic Willard Hotel, which has been vacant since 1968.

Pennsylvania Avenue is to be redeveloped. After 15 years of planning and controversy, Congress has granted an initial $29 million to revitalize the avenue and the surrounding area. President Carter signed a measure including the funds last week. Yesterday the government-sponsored Pennsylvania Avenue Development Corp. held a news conference to mark the long-sought breakthrough.

"This fall, you will see some visible signs of our effort," Elwood R. Quesdada, the development corporation's chairman, said during the news session.

Quesada and other officials promised a cornucopia of welcome changes: tree-shaded walks, public plazas, new housing and shops, preservation of historic buildings, lessensed traffic congestion, a revival of downtown nightlife, and an influx of thousands of new residents to Washington's long-deteriorating central strip.

The rejuvenation of Pennsylvania Avenue, nonetheless, is expected to take many years. "I think an optimistic number of years would be 10 and you shouldn't be surprised it it would be 15 (years)," Quesada said.

The Willard Hotel is entangled in a court suit. Other property in line for redevelopment must be appraised and taken over by the Pennsylvania Avenue Development Corp., which then will lease it to private developers. Bids must be solicited, contracts negotiated, engineering studies carried out and architectural plans drawn up. Zoning restrictions and the city's controversial rent control law are among other issues confronting the developers.

In all, the Pennsylvania Avenue Development Corp. hopes to spend $130 million in government money and to persuade private developers to invests another $400 million to $500 million in the area, a spokeswoman said.

The fall planting of willow oaks is expected to be the first sign of change along the avenue. Three rows of trees are to be planted on the north side of the avenue and two rows on the south side near the new National Gallery of Art annex, at 3rd Street NW. Also the sidewalks there will be widened, officials, said.

Later this year, another change in Pennsylvania Avenue will begin to take place between 13th and 14th Streets NW. A public square and fountain are to bre created in the center of the avenue, with traffic to be routed around them. This undertaking may take two to three years, the corporation's spokeswoman said.

The first structure along the avenue expected to be taken over by the development corporation is the beaux arts Willard Hotel, at 14th Street NW, which was built in 1901 and has recently been the subject of a U.S. Court of Claims suit. The hotel's restoration is thought likely to cost $17 million the corporation's spokeswoman said.

The Pennsylvania Avenue Development Corp. has already had what officials described yesterday as serious discussions with three films interested in refurbishing the Willard. One, they said, is Oliver T. Carr Inc., a major Washington developer. The other two are out-of-town companies whose identities were not disclosed.

Among the next privately owned properties to be acquired by the development corp. officials said yesterday, would probably be the sites of the defunct Lansburgh's and Kann's department stores. They will give way, the officials said, to a "superblock" between 7th and 9th Streets north of Pennsylvania Avenue - an unusual complex of offices, shops, high-rise apartment buildings and small row houses.

All along the historic avenue, development officials appear to be searching for a touch of elegance, including such architechural concepts as terracing. "It would be a very lovely concept," Quesada said yesterday, "if it can be implemented."