wow thats a really general question. You didnt mention what operating system you use but i'll guess you have windows xp since this is the windows xp forum. You also didnt mention what kind of phone you had - Android, Iphone, blackberry?
You probably want to install a vnc server on your windows xp computer and a vnc client on your cellphone. that is all there is to it.
how can i control my computer with my cell phone