Its fair to say that much of todays world is online, school, work, doctors appointments and parties. Everyone is on the internet. It's your job to find the people that want to buy your product or service.
Of the all the websites they're on which one is the place you want to introduce the idea of your business?
The answer completely depends on what you are selling and who you're selling it to.
Its easiest to sell things that you actually like and use. The best market research is looking into the things that already interest you from a different perspective.
What products do you use? Body care, clothing, food etc. these are all things that you pay for (or are bought for you) that you could be getting paid for. There are many options in all of these categories, and there is a reason that you choose the ones you do. Are you guided by environmental issues, low cost, high quality or usefulness? Your customers also care about these things and make their own decisions. Give your customers a reason to choose your business over the other one.
If you are in this program you are starting your business at a younger age than most. You're getting a head start that is valuable in the long run and people want to support a success story. That said, people will only give as much to you as you give to yourself so invest the time into your business by answering the following questions.
What makes your product unique?
Who understands and appreciates this unique quality? And where do they spend their time on the internet?