Take the screw in part to home depot and find a size reducer in the plumbing section. If you can take the one off your faucet with you it will simplify things. I had to do this on mine as well.
Home depot is where you'll find what you need there is so many adapters to get that you'll find one and if you don't go back and ask for help if they for some reason don't have it they should be able to order one.