You can set up a website, gradually build up the content (articles, videos, podcasts, etc.), then eventually monetize the site through advertising, affiliate marketing, or even the direct sale of specific products or services. Even better, you can generally find whatever services and technical assistance you need online and free of charge. Later on, when your site develops a reliable cash flow, you can begin working with paid providers who can take your blog to the next level.
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you. 
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