If you don't already know what niche or market you're going to be playing in, then this is one the first things you'll want to determine.
It's important to realize that niches are "needs" (of your prospective customers). They aren't just something that you pick based on what you think might be good to enter. You have to first determine whether there is a real "need" in the market place for that particular product.
One of the best places to get ideas for a niche is the Amazon.com marketplace. You can determine quickly whether or not there is a need for a product.
You can do this by first "shopping by department" and then selecting a category. There are hundreds of categories and sub-categories to choose from, so I would suggest starting with the top 3 categories that you feel drawn to -- preferably evergreen type categories like "health and beauty". You can select others as well later, but this should give you a great starting point.
The key is to drill as deep as you can into the category to check to see which niches are viable. Once you get deep enough into a category, you can start evaluating the top products to see what's selling. It's important to note the quantity of reviews and ratings to see how well the product is doing. You can also check how well the product is selling by clicking on the product and navigating to the Amazon "Best Sellers Rank". You'll want to note the average customer review, ratings, and best-seller rankings for the top products in the niche.
Also read the reviews to determine what people are saying about the product and where it could use improvement. This could be an opportunity for you to create something that fills that need. Do this for each of the top products in your niche.
If you see that the sales rank for the top products in the category are too high (not enough sales) then it might not be a viable niche to enter. However, if you notice that there are many products that are doing well with less than 100 in the category, then it might be something worth considering. If you're an aspiring author or coach and plan to sell digital products (ebooks, online courses, magazines, etc.) then you'll want to check out the best-sellers kindle list. Here you'll find what kind of books are selling.
Select one of the categories from the left (i.e kindle ebooks) and then drill as deep as you can into one of the primary categories (i.e diets & weight loss). There you'll see which ebooks are selling the best. It's important to go as deep as you can into the categories because that is where all the real action is. If you target something too broad (i.e health), then it'll not only be too competitive, but you'll have a hard time selling your product or ebook to a target audience (and ranking well if you publish a book to Amazon). Take a look at the reviews (both number of reviews and ratings) and check the sales rankings. You can determine the best sellers rank by clicking on the book and navigating to the product details. This will help you determine the potential of the niche. If you notice many of the books on the first page have a very high sales rank (greater than 50,000+), then it's likely that there's not much activity and sales being made in this niche.
On the other hand, if there is a very low sales rank (less than 10,000), then it may signal that there is very good potential and sales to be made. It's important to go through all the books on the first page to get a feel for the pulse of the market.
Read the reviews -- both positive and negative. See what people are saying. See what challenges they had and what you could potentially improve on. It's important to write all this down in your ma