Practice, practice, practice.
Start moving forward slowly in a low gear and then pop the front wheel off the ground by suddenly pushing a pedal forward and pulling up on the handlebars at the same time, and just keep pedaling while trying to balance on one wheel. If you accelerate to a point where you cannot pedal fast enough to maintain balance on one wheel, shift up 1 gear and keep your momentum going. Keep upshifting when you can't pedal fast enough to keep the front wheel up, and eventually (it took me about a year or two) you'll be able to cover the length of a football field on one wheel while accelerating thru several gears on a single wheelie, all the while anticipating and counteracting all the little obstacles (bumps, rocks, holes, tree roots, curbs, etc.) that you see on the ground along the way.
"Doing wheelies on a long climb is a great way to practice while reducing the monotony. And you don't have to use the brakes to slow yourself down for the next wheelie." <--This is great advice. On a grade that's not too steep going uphill, you can upshift, downshift, or stay in the same gear and just try to keep the front wheel up throughout the entire climb. Eventually you'll be confident enough to steer around objects by leaning to the left or right while riding a wheelie and even use just one hand on the headset or on the handlebar and ride that wheelie to the top of the hill while tipping your hat with the other hand. I'm in my fifties now and totally out of shape, but in my thirties I was able to regularly wheelie up a hill that was a quarter-mile long, but because I never learned how to properly feather the brake, I had to accelerate to keep the front wheel up as the hill flattened out at the top. The first 3/4 of the climb was a monotonous single-gear slow wheelie up the hill, but that acceleration at the end made it a stomach-crunching workout every time.
Different frame geometries call for different weight distribution, so if you're not have any luck after a few weeks, try changing the stem or the seat height.
Practice, practice, practice, and you'll get there. Try again and again, and if you fail, just remember: there's no shame in failing because at least you're trying. "If at first you don't succeed, try, try again." If you fall, get back up, dust yourself off, and try again, no matter how many times you fall. Do this and you WILL get better and better over time. If you quit, however, or worse, if you never even try to begin with, then you'll never get there.