Hello:
Go to
ODP - Open Directory Project. Why? There are over 500,000 niches right there! While there are lots of things to know about affiliate marketing, the different variables, finding niches is relatively easy. Although the categories start out broad, to answer your question, you don't want to go for the larger niches but rather the smaller sub niches. Here is where you get your smaller groups that are better to target because these groups are looking for much more specific items to fill their needs. They are much less competitive than broader larger niches.
At dmoz.org if you click on a category instantly you may have tons of sub-niches in front of you. If you don't then it's probably a good bet that this broad niche may not be a good choice but if you find lots of sub-niches then that alone tells you that there is lots of interests in the smaller sub-niches and so you dig a little further. O.K.
I just clicked on computers at dmoz.org, guess what? Instantly I found well over 50 sub-niches just like that. Some of these sub-niches have anywhere from a couple of hundred to tens of thousands of mini niches in them. What do you suppose that means? These are less competitive products that have their own audiences that you know are very active. Now how long did that take? Anyhow I went a little further I clicked on computers, software tons of stuff there, graphics. There I found a category for 3-D graphic tools!!!
After clicking on several of the 3-d graphic tool selections and running them through google until I found a product Digital Light and from the drop down from that several times I found "digital light meter for photography". There were 6-7 sponsered ads for this product. I'll monitor this for a few days to see how the ads do and if they stay or increase I'll have a good idea of whether I want to see what it sells for or if there are specific brands of this product that sells good etc. If the commission is good and are handled by a reputable source I may build a page around it targeting the audience that is known to use this item.
Point is you can find very profitable sub-niches and mini-sub-niches and target audiences that have very little competition and earn great money. Don't bog yourself down making it harder than it has to be. Keywords? go to Seodigger.com after you have a couple of top SE placement sites pushing this product, type in the site and look at the keywords that they rank for to get on the right track for keywords to rank for for your page.
Cheers!
salamei