When you are just getting started online, the very first thing you want to start doing is building a list. This is how to make money online the fastest way that I know of, and really, the only way I recommend people get started. If you don't have a list of subscribers, people who listen to your recommendations and are open to taking advice from you...you really don't have anything. If you do, then you are in business.
Now there are things you can do with that list. You can market products or services of your own, or, if you don't have any of your own yet...you can become an affiliate of someone else's product and market that.
The main thing to keep in mind is that you promote things that people on your list are truly interested in. I know this probably goes without saying, but a lot of people seem to miss this point. For example, if you are in the tennis niche, you wouldn't send something golf related. Even though tennis and golf are both sports, it is unlikely your tennis customer will be interested in a golf item.
Obviously if you are marketing a sports drink that keeps your body hydrated for longer endurance, it would fit for both tennis and golf. Bottom line here, use your head...plain old common sense goes a long way in the internet marketing world.
If you need to find affiliate products that would be a good fit for your list, there are a few things you can do. You can go out to ClickBank and search the tons and tons of products they have. Just type in the keywords that relate to your niche and see what is available.
You can also simply go to Google and type in your keyword and the word affiliate, or affiliate programs and see what comes up. So in the tennis example, you would type in tennis affiliate program. You will get all sorts of results that you can take a look at and decide if any of them are right for your list.
Make sure you check out what you are going to promote, and by this I mean you really should buy the product so you can evaluate whether or not it is a good product. When you are checking it out, try to look at it from your customer's eyes. What would they think about it? Will it help them? Or solve a problem they have? Or make things easier for them? Be honest about your evaluation.
If you do this, you will end up with a list of very satisfied customers who will love to get information from you because you always give them good stuff. They will grow to trust your recommendations and view you as an expert that can really help them...and that is really how to make money online...really the best way to build a profitable online business.