One of the biggest mistakes that new affiliates make is create a general website and plop all the banners they can find on it and then expect the cash to start pouring in the next day... of course it doesn't work that way and that is where affiliate marketing loses alot of people.
Do not expect to join an affiliate program today and be able to quit your day job tommorrow. Affiliate Marketing takes time and ALOT of patience.
I always tell newbies to find a product/service they can relate to and would know alot about and then build a theme park around that... for example, a car buff would know alot about cars and automotive stuff. So they could create a car related site with car care tips and any other information they could give anyone interested in cars that would come to their website and want to see. Then once they get their site going with alot of information they would find affiliate programs that sell car related services or products such as insurance, auto parts and accessories, etc...
Just my two cents... hope I am not to late