"We've been doing seminars with the family just to educate them," the 43-year-old star of the upcoming "Mission: Impossible III" film tells GQ magazine in its May issue, on newsstands April 25.
"Running seminars so we can understand what Kate's going through, and for Kate to understand it. Things like how to take care of a pregnant woman and get ready for the birth.
"It's just kind of becoming a fun game of learning. We've also been studying what happens after the birth and how to take care of the baby."
Cruise says his children, 11-year-old Connor and 13-year-old Isabella — from his marriage to Nicole Kidman — will help take care of the baby.
"The kids will have different responsibilities and run in shifts. They're going to help on every level," he tells the magazine.
Cruise and Holmes, 27, have been engaged since June. Her pregnancy was announced in October.
The couple haven't decided whether Holmes will give birth in a hospital. "At home, maybe," Cruise says. "We're really going back and forth on that one. But it's gonna be a blast."
And they are remaining silent about the sex of the baby.
"We haven't told anybody," says Cruise. "It's either gonna be a boy or a girl."
He joked that a sonogram machine he bought when Holmes got pregnant is "strapped to her 24 hours a day. Absolutely, anywhere she goes, we have it attached." (They plan to donate the machine to a hospital.)
Cruise declined to comment on whether he called Kidman to tell her about the pregnancy.
"It's not like anything I had to or didn't have to do," he says. "I wish her well, and it's cool. There's just no issue with any of it."
As for a wedding date, "We're kind of looking at late summer, early fall. I always thought it was going to be like three days and have it be a party. But now I don't know, because all our attention is on the baby and the movie."