How do I clean my GPU cooler / fan?

I have a Radeon HD 4650. Its temperature skyrockets to over 100 degrees celsius when I start a game. I think it's because I've never cleaned the fan/cooler ever since I bought the machine two years ago. I am wondering how do I clean it, and do I need a thermal paste (which i don't have) as well? If so, where do I buy it? I live in Canada and I'm pretty sure retail stores don't have it.