How can i Change my domain?

I am from Bangladesh and i have created a website in some free hosting site (xotil.wall.fm) now i want to buy a .com domain. if i buy the domain named (xotil.com) how can change my site address. i mean when user will enter ulr xotil.com the page xotil.wall.fm will open. I will appreciate if any one can help me with the answer.