Virginia Gov. Charles S. Robb, with two years left in office, is facing the most intense and crucial decision-making period of his administration.

In coming weeks, Robb will try to place his personal stamp on the state's government, an action that could help determine whatever future role he hopes to play in national politics.

"This is his big shot, our shot at the ball," said Human Resources Secretary Joseph L. Fisher, a member of Robb's cabinet and one of the top officials who are advising Robb on the decisions.

"There's no question that it is probably the most important single legislative period" of the administration, Robb said in an interview Friday. "I don't like to get too much hype into it. It is important."

In the weeks leading up to the General Assembly session that begins in January, Robb must:

* Put the finishing, politically sensitive touches on his first two-year, $14 billion budget and any possible new taxes to support it. It will be the only budget prepared entirely by his administration that the legislature will consider.

* Decide details of a major government reorganization plan, under study for months, that could change how his cabinet and state bureaucracy is run.

* Draft dozens of legislative initiatives on a wide variety of topics, including drunk driving, regulatory reform, education, and voter registration.

* Lead the state Democratic Party into the quickly developing presidential campaign and pump up the party's lagging efforts to get behind a strong candidate to oppose Republican Sen. John W. Warner next year.

Although Robb will have one more session in 1985, that General Assembly is expected to be dominated by legislative leaders who want to run for governor or other state offices; Robb cannot succeed himself.

The cumulative effect of the governor's current decisions, according to interviews with several administration and legislative sources last week, will have a major role in determining his place in the history of the state.

Although one senior aide insisted that the coming weeks are "not the end all, be all" of the administration, another said: "Robb will be tested as a public leader. Not necessarily that he wins everything, but how he controls himself, who he is and how he'll be remembered, and what his future will be."

Robb's actions also will help decide, officials say, whether he can position himself for "lightning to strike" him as a vice presidential candidate next year or possibly for a cabinet-level position in a Democratic administration.

A Robb campaign against Warner next year is still seen as possible, but not very likely, these politicians say.

Until now, Robb has not been able to concentrate fully on the coming issues because of his extensive travel, championing Democratic legislative candidates around the state and promoting Virginia recently in Europe and Japan and to Wall Streeters in New York. He also is head of the Southern Governors Conference, which met in Austin, Tex., last month. A meeting in Williamsburg next year already is being planned.

A myriad of decisions are expected to come in rapid-fire order--but not necessarily be announced--after the Nov. 8 General Assembly elections.

Staff members have been putting together detailed, but closely guarded, option papers for Robb. His six-member cabinet is to meet Sunday in a daylong session to thrash out remaining issues on reorganization.

Major tax and revenue decisions are due early in December.

In fact, Robb's plate is so full that Sen. Edward E. Wiley (D-Richmond), the influential chairman of the Senate Finance Committee, has cautioned the administration against trying to do too much during a legislative session that is certain to be dominated by budget questions.

"I've asked them to consult with us so they won't be disappointed," Wiley said recently.

The crucial decisions Robb must grapple with arise from a state law that forbids a governor from succeeding himself, and Virginia's system of two-year budget cycles.

Robb, elected in 1981, came to office facing a budget prepared by Republican Gov. John N. Dalton. He offered some minor revisions and then spent the 1982 and 1983 legislative sessions fashioning his own priorities in education and economic development from Dalton's outline.

In his first two years, Robb generally is given high marks by friends and foes for managing the state's budget during the recession, when he slashed state spending to avoid deficits.

Robb aspires to more, however, than just being the governor who guided the state during difficult economic times. In the short memory of voters, such accolades quickly could fade if the economy continues to improve and the demand for services rises. And Robb administration officials believe that they have done all they can do to cut the budget.

"I do expect, with each round of cuts, there will be screams and I hope that we have had our last," said Secretary of Administration and Finance Wayne Anderson. "The governor insisted we would wring out new programs and not be condemned to status-quo qovernment."

The key will be the revenue forecasts that are made in December. If the projections are up, Robb is expected to unleash major initiatives and cancel some budget cuts now in place. If the forecast is for sluggish times, it will mean another make-do legislative session, with just a few programs in education and economic development likely to win approval.

Robb, who is expected to televise his State of the Commonwealth address again this year, has steadfastly said that he is not considering tax increases. Virtually all of the officials agree that any new tax increase will have to come this session, rather than during the 1985 gubernatorial session.

George Stoddart, Robb's press secretary, insists that "if there is any tax change it will be done because of revenues, not because of any politics.

The question is whether you need an increase or not."