Here is some more information:
One thing that is neccessary for a successful business is a popular niche. I divided them into 3 categories that are most successful. You want to focus on the sub niches of these to make it easier for you.
Health (Everyone wants to be healthier)
Wealth (Everyone wants to have more money/material goods)
Relationships (Everyone wants better relationships with their family, friends, and partners)
Health: weight loss, health and fitness, muscle gain, anti-aging, acne, hair loss, Body building, mental health, halitosis cures, etc.
Wealth: Home Businesses, Credit Problems, Gambling, Internet Marketing, How to avoid foreclosure, How to avoid bankruptcy, Investments, stocks, etc.
Relationships: Dating, Wedding Planning, divorce, How to Have a Better Sex Life, How to Get Back Together With Your ex, How to Seduce, etc.
Obsessions and hobbies are also things that people eagerly spend money on like clothes and gaming.
Also to find out what's popular in your niche check ebay pulse, & Amazon movers and shakers.