If you want to start a niche site and make money online, you have to think about a few things. For instance, how to select a domain name suitable for your niche, how to select an affiliate program matching your site, but I think the most important thing is how to start working on getting traffic for your site.
In order to get more traffic, firstly, you must write high quality content to attract visitors to visit your site frequently. Then setting up an email newsletter is also good way to get traffic. Lastly, submit your site to directories and the search engines.