“I feel for the first time like we are actually going to beat some of these problems,” says Matt Damon, 35, who recently embarked on a six-day “listening and learning trip” focusing on AIDS-ravaged Zambia with his brother Kyle, 38. The mission, organized by and Bono’s activist group DATA, left the Good Shepherd actor and dad-to-be feeling inspired. “The world I leave to my daughter is going to be affected by what I do, or what I don’t do,” he says. “The best way to honor and love her is to do what I can, here and now.” Damon talked to PEOPLE about what he saw, how his life has changed and how he’s getting ready for “zero hour” – the arrival of his first child (due in mid-June), with wife Luciana.

You have a reputation for being low-key. Why did you want to go on this trip?
It was the best thing I’ve ever been able to do because I am a celebrity. It was kind of like going back to college. As an American I knew I’d be seeing things you don’t see every day here.

It seems like the whole Ocean’s Eleven crew has made this trip to Africa. Did you ask them for advice?
I called George (Clooney) before I went because I knew he was going into Chad and that was a very risky trip he was taking. It’s going to be interesting when we all hook up to recap. I’m looking forward to that conversation.

What one image from the trip remains with you?
This one group I met at this clinic was encouraging people to divulge their (HIV) status. A lot (of infected people) don’t get medicine because they are afraid. These people wind up on death’s doorstep, and they’ll go and get the ARVs – the anti-retroviral drugs. These things have the Lazarus effect on these patients. Within six months they look fantastic. All of these people in this group were alive because of American money that went there. So that felt like, “Wow. It’s working!”

This seems to have touched you very deeply.

It changed my life. It’s not a trip that most people will get to take. I think if every American saw what I saw, they would want to help. Everyone has their own problems, their own lives. I have my kids, I have my bills. People’s lives are full. I think they want to help, but they don’t know how to do it in a way that won’t totally consume their lives. ONE is a great way to do it.

What difference do you think you can make?
I’m sure some people will dismiss it as a celebrity talking about something they don’t know about. And some will listen to what those of us who have been over there are saying. I just know it’s important to me to do the best that I can within the sphere of influence I have.

Will you go back?
I plan to go a lot more … Obviously right now, I’m here till zero hour. I’m not going anywhere.

When is your wife due?
Good question. We’re due in mid-June, but who knows? We could be now.

Your daughter and Ben Affleck’s daughter Violet will be practically the same age. Are they going to have playdates?
Yeah, they will. All we have to do is end up living in the same city!

Is your stepdaughter Alexia, who’s 8, excited to have a little sister?
Believe me, if it was a boy it would have been a big problem. She wasn’t having it. She definitely wanted a sister.

Good thing you planned it so well!

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