I wouldn't rush to buy new tires. You'll just spend a lot of money and still be struggling with the hill. IMHO, just keep at it. Give your body some time to build itself up. Ride each day or two. Hit the hill at a slow and steady pace. See how far you can get each day before stopping for you first break. Do this for a few weeks, maybe a month or two, and I will be surprised if you do not see notable improvement.