There should be a bladder inside the pontoon. Take it out, locate the hole, and if its small, you can buy a pontoon bladder patch kit. Just follow the directions. If its big, I would try and get a new bladder. I'd hate to be out on the water and have the repair fail. Also, if the failure is at a seam, that can be a little harder to repair.
Creek Company Patch Kit (LINK)
Creek Company Replacement Bladders (LINK)
Not sure which exact pontoon boat you have, so couldn't get you the exact page for the replacement bladder, but scroll through that 2nd link if that bladder has a big hole.