Is it Easy to Apply for Patents?

I have just come up with a great variation for a product that is already on the market. Can anyone tell me how hard it is to apply for patents? I am hoping it is pretty simple and that I will just be able to do it online without spending a ton of time on it, but I am sure that there is probably a bit of work involved no matter what. I just want to make sure no one has thought of this before, and, if not, then I want to file for a patent. Afterwards I am hoping I can sell my idea to the company that makes the product now.