There are many businesses you can choose from. What business you choose depends on several different factors. So if a person who is successful in real estate, you might not be as successful in the same niche.
1- Talking about online businesses, blogging is said to be the most successful business. Not only that it is very easy, but it is indeed very profitable.
2- Then we have affiliate marketing. Affiliate marketing does not need any investment, however if you want to have your own blog or website that is good, otherwise affiliate marketing can be done in several different ways.
3- You can sell stuff on eBay. Again it does not need any investment. All you need is something that you can sell and others are willing to buy it.