You need trial and error
Some webmaster started in internet ventures expecting to strike the jackpot at first try. Fact is most newbies will need trial and error until they find the right stuffs that are making money for them.
I’ve built over 20 websites in different format and topics, and so far only 2 are considered really successful, while another 3 are still worth working with.
That gives me roughly 20-25% of success rates in my online ventures, and I believe I’m doing ok compared to many others.
There are some webmasters that have higher success rates in their business ventures, luck plays a part but note that most of them did some thorough research on a specific field before they started something.
So unless you are a really creative person or have the expertise in some specific topics and products, you will need trial and error.
Depends on your time allowance, start a few different projects at the same time. Find the right niche before focusing on a real profitable venture.