Topics like health, internet marketing, gambling, etc, are all fiercely competitive, and very difficult for inexperienced marketers to make any decent money from. You can of course target these markets over time, but I recommend finding your own niche first and marketing to those people in these more narrow markets. These people are most likely not yet over-exposed to traditional marketing methods associated with internet marketing like squeeze pages, opt-in forms, sales letters, and so on, so you are more likely to be successful.

A potentially profitable niche would be one that has a decent number of keyword searches conducted every month, I would say at least a few thousand, and one where you could potentially achieve the number one spot, or at least get on the first page of results, for this keyword with a pagerank of 5 or less. This is Google's measure of how many links and the quality of links that a site has, and although it is not the only deciding factor, a higher figure will generally results in a higher ranking.

If you carry out a good link-building campaign, and fully optimize your website, it is not too difficult to achieve a pagerank of 5 or more within just a few months, and start climbing up the rankings. However, you have to remember that it can take several months for this high PR to be rewarded with a good ranking, particularly with Google and Yahoo.

To begin thinking up potential niches I recommend starting off with a blank piece of paper, or a blank page on your computer, and start brainstorming. Start listing all the things you're interested in, and also break these down into smaller, more narrow subjects as well.

You can also find out what other people online are talking about. Visit blogs, article directories, forums, groups, etc, and see which subjects seem to be cropping up over and over again. Again list any related keywords you can think of. You never know, one of these may be a potential goldmine.

Next take a look at the book and magazine sections of Amazon, and see which type of publications people are currently buying by looking at the top 100 or bestsellers list. Similarly you can go to Ebay and find out which items are selling more than others, and are therefore currently in demand.

Also look around you, closer to home. Visit libraries for potential ideas, and also go into newsagents, and look at the magazine section. If there's a magazine on a particular niche subject then there must be a decent demand for it, otherwise it wouldn't be in circulation.

Just be creative and you will soon have a large list of potential subjects and related keywords. Once you have this information you are ready to start conducting keyword research, as I mentioned earlier. There are various tools available, both free and paid services, but a good place to start is Yahoo's search marketing site. Just keep entering the keywords you've gathered and you will soon uncover high demand, low supply keywords that you could potentially base a website on, and start making some money from.