Overall this is good advice for anyone, so I think it's the best first step for anyone who's inactive or overweight. I'm not a doctor but I suspect this is how a doctor would approach it too.

I know it's anecdotal, but of my friends with fertility issues (3 couples who mentioned it to me), none are remotely overweight, they're regularly active, and they eat well. I'm sure inactivity and obesity are a major issue in regards to fertility, but I'm not personally seeing that.