Here is the critical question? Do we select a product that we like or do we select a product that is highly demanded in the market.
Actually the answer is both as in order to market a product successfully, you have to believe in on one hand and you need to choose your market carefully on the other hand.
Over the web, there are millions of products with a very high competition. thus you need to get ready for this. As the studies indicate the highly niches on the web are under 3 main popular categories including health, wealth and life style. However even under those categories, there are thousands and thousands of products and that's why you need to be so selective and creative.
You can build your own product as well, but this may take you time and efforts. Therefore, my recommendation is to start your web business as an affiliate, then there will be a time when you feel that you are ready to create your great product using the appropriate tools.