Do not fall for those business opportunities that promise you will make $1000 in one day. Oh, please! I am so tired of seeing those, and I wish there were a way to ban the lying dirtbags from the Internet. But that is the beauty of the Internet, it is free enterprise, so it is by nature a buyer beware market. And don't invest money in ANYTHING until you have thoroughly checked out their credentials, talked to others who are in the same business, and know that it is something that gets you excited.
Finding a niche that is not diluted is the main problem I see. For instance, a guy paid me $15 an hour to give him my opinion on his new business website. The first thing I did was investigate similar niches. I quickly discovered that there were at least 4 other people doing the same thing he had in mind, and doing it much more effectively than what he was proposing. First of all, he was charging a subscription fee and giving nothing away. And worse, you had to go through pages of the website and still you could not find out how much the subscription cost without giving your email. I told him that if I were a potential customer, I would have left without ever doing that. Customers want free stuff, even if it's just a newsletter. You have got to give in order to receive, especially in the beginning.
A good place to start if you are trying to decide on your niche is to check out some Internet Marketing Forums--Search Engine Journal has a list of what they consider the best ones--and click on the signatures of some of the members. They are looking for affiliates left and right. But as I said, just be careful, and make sure you do your homework.
If you follow this advice, you will be able to find your niche and be thrilled at the prospect of having your own exciting and successful online business.