Well, If you have something that interests you like a product, Do some research, Use free keyword tools to give you an idea on how well they rank etc. Do some research yourself by putting the keywords in Google and see what pages pop up and how your competition is. See if there is an affiliate offer for your product and make sure people are buying it. Then try making your website.
Its about research and then applying what you have learned. Trial and error. Don't give up if you fail the first time.