Rep. Steny H. Hoyer (D-Md.), with an entourage that included his wife, Capitol Hill aides and a personal friend, was more than ready yesterday to take a small step into the political big time as the Democrat chosen to respond to President Reagan's weekly radio address.

"This will probably be Reagan's basic New Year's message," said a congressional aide who usually accompanies the Democrat annointed to give the five-minute radio response at the ABC network studios in Washington.

"Yes," said Hoyer with a grin, "and ours will be the basic New Year's 'You ain't right, Mr. President' response."

Hoyer follows a long line of Democrats chosen by House Speaker Thomas P. (Tip) O'Neill Jr. to deliver the Democratic talk. But for Hoyer, just elected to his second term, it was a rare opportunity for national publicity--and an example of Hoyer's ability to win favors usually reserved for more senior members of Congress.

The Democrats never know what the president will talk about beforehand, and yesterday was no exception. "Oh, no, not drunk driving," Hoyer's friend, Lanny Davis, groaned as the president's message was broadcast.

The voice of Ronald Reagan filled the little conference room where the Hoyer party listened to the broadcast and composed its response to be aired one hour later. Reagan noted that 25,000 Americans die each year and 650,000 others are seriously injured in alcohol-related traffic accidents.

"The personal pain and heartaches caused by these needless tragedies is immeasurable and billions of dollars are lost in medical costs, wages and through hours of missed work," Reagan said.

Reagan mentioned Maryland as one of the states that has intensified its programs to fight drunk driving. "Highway deaths there are at a 19-year low," he said.

" . . . If we insist long enough and loudly enough, we can save lives. So I thought it appropriate to start the ball rolling on this, the first day of the New Year."

When the president finished, Hoyer and the others took the four-minute speech the congressman had prepared and began to tailor it to the occasion. At 1 p.m., Hoyer entered the broadcast booth and announced that he agreed with the president on "the critical problem of drunk driving." Then Hoyer quickly turned to the Democrat's favorite subjects: Reaganomics and unemployment.

"The president has made the multimillion dollar MX missile his top priority for this year," Hoyer said. "He has established a bipartisan commission to reach agreement on solutions to the missile-basing question."

"I call on the president to establish a bipartisan commission to address what most Americans consider to be their number one priority--jobs--to put the 13 million unemployed Americans back to work." Any number of Democrats, Hoyer acknowledged, could have delivered that message.

"From an ego standpoint, I'd like to say they picked me because I was going to do a great job. But practically speaking, I'm right here and everybody else is out of town," said Hoyer, who lives in Prince George's County.

"Then again," said Hoyer, a savvy politician with his eye on an eventual leadership post, "I'm sure they wouldn't have asked me if they thought I was a dunce."