Assuming you're refering to the hole in teh propeller..

You have to buy the right crankshaft for the prop.. Hahaha !! No, No, no..

You need to 'ream out' the hole. You can get a special 'prop reamer' for the job (not a bad Idea..). I beleive Hobbico or Great Planes have one. You can get a mechanic's 'tapered reamer' (and you should be able to pick one of those up at teh local car parts store.. ) but will leave a tapered hole - not really a bad thing. Or, you can drill out the hole with a drill press. Don't use a hand drill, as you can angle the hole, make it off center, etc... go in small steps, adn go slow!

Since you're posting in the beginner' section, you might want to look up a local club mate (your instructor for instance) and ask him / her if they have one, and can do it for you... Actually, show up at teh feild all ready to fly, and someone is bound to have one / be willing to help you out!

Balancing the prop should be the next step, after it fits!