Affiliate marketing is not an easy business, particularly if you are trying to sell a product from Clickbank or Amazon, that has a good commission, because a product like that will obviously have a lot of competition.
Affiliate marketing has a failure rate of about 95%, because people try to use the same methods as everyone else, given to them by "gurus" who are just trying to sell you their "secrets" for a fee.
You can make a niche website if you want, but I suggest you write an article or two about the subject first, on a site like Triond, and see how well you can do in a Google search for many different article titles about the product.
You can put Clickbank or Amazon links into your articles there, with Amazon, you have to get the html text link from the product, and highlight the link between the first two quotation marks.
Once you have discovered whether the particular affiliate product has competition that you might be able to push out of a Google search, you can pay for a domain name, and hosting, and build a whole niche website about that particular product, if you think it's worth it.
As far as SEO goes, I've written about it before, and the SEO for an affiliate product isn't that much different to the SEO for anything else, you just try to build a bigger, better, more relevant site with more back links etc. Check out my article: What do you Need to Know About SEO?
The real question is, where do you find the right product to become an affiliate for? On Factoidz, you are given products you can write articles about, if you want, and can get a bonus of up to $20 for a good article, and high rates of pay for the traffic to them.
Personally, I like to be an affiliate for free to join sites, like Factoidz, and many others, because I believe that you will get ten times more people to click on the link if they think it is going to make them money, and not cost them money, and then ten times more after that who sign up.
Selling jet lag remedies, lipstick, and baby clothes just isn't my thing, and after taking a good look at what is out there, I don't bother with Clickbank and Amazon anymore. Affiliate marketing is all about finding a niche, and you can bet that every profitable niche is pretty much all sown up on those sites, by the big boys of affiliate marketing.
Have a go at it if you want, particularly if you have some money to play with, and try writing reviews for products at Factoidz, but I would recommend you don't go into it without a good knowledge of SEO, and a truly unique product to be an affiliate for. Pay per click is always an option, but it is even more risky than paying for a domain name and hosting.