There are no overnight success stories anymore: This is probably the hardest pill to swallow for those looking for quick success. Just like any other business in the brick and mortar world, success online is not an overnight phenomenon. Longevity of your web-property and your online business builds credibility, trust and a name brand. These are important ingredients for success.
Traffic does not find you easily (at least not cheaply!): Once you have built up your web-property and are open for business, you need individuals to visit your site. In the offline world, you depend on your location (say in a mall, or popular street), your signage (that says what you are offering) and any advertising/promotion campaign that you undertake. The situation is very similar in the online world - except that malls and main streets are not so common. The equivalent sources of free traffic in the online world are Natural Search Engines like Google and Yahoo!. If a user, upon searching a term, finds your web-site in the results pages, and chooses to click on the link to your site - you have a free visitor! But there are too many "ifs" here: You must be offering something that a user wants. Your website has to show up in the first 1-2 pages of results (in direct competition with others who offer similar services). The quick summary that the results page shows must be enticing enough for the user to click on. And so on. There is a lot of effort that goes into tuning each of these steps before you can hope for some free traffic through natural search engines.
Content is king online (repeat: content is king online): Many website owners are onto this now, and tend to pack their retail websites with content. The Search Engines are in the business of providing answers to user's questions. Other websites also like to link to sites that are offering solutions to their visitor's questions. Almost no one likes to link to retail sites (unless they are affiliates or are linking to some hard to find products). This means, even if you have a small array of products to sell, you must provide content. And more content. And update frequently. It takes a while for this to sink in - because adding content when there is no traffic to your site requires huge doses of motivation. But keep at it.
Start simply: There are many techniques for making money online, including membership sites, product sites, affiliateships and so on. The simplest method is to create a content-rich site and monetize the resulting traffic through Google AdSense, or Yahoo! or MSN AdCenter. It is highly recommended for a person new to web-business to eschew all other means and to start by creating a simple content website and monetize through advertising. This gives you the right experience with designing a site, creating valuable content, and in building traffic. This experience is irreplaceable and gives you a sense for how the web-world works.
Build Slowly: Once you have a basic content site in place and are monetizing some of the traffic through advertisements, add some relevant affiliate sites. Becoming a successful affiliate is an art unto itself, but having learnt the art of bringing traffic to your site inexpensively, you can devote yourself to becoming a profitable affiliate of 2-3 partners. Do not over-extend yourself in this effort until you master this art thoroughly with a limited number of partners.
Working in Concert, and never quitting: The final step is to make all your web-properties synergistic, each bringing traffic to the other, giving you insights and allowing all of them to prosper. Importantly, do not quit on your overall goals. You may switch ideas, but once you believe in what you have, keep at it and keep learning. Success will come.
Best of lucks!