I'm not going to post a detailed guide, but I can point you in the right direction, so...
Start with keywords that have high CPC. Use your favorite keyword tool to check their CPC and when you find 10-20 high CPC keywords (high CPC is subjective and it's up to you to decide what CPC is good enough for you) in one niche go with that niche. But beware, high paying niches are also competitive, so you will have to compromise at some point and either go for niches that don't pay much but are easier to rank, or niches that have high CPC but also a lot of competition.
The high paying niches are usually those that are expensive (duh!), such as...
- medical treatments (surgeries, dentistry, etc)
- luxury goods and services (hotels, resorts, cars, yachts, whatever)
- everything that involves money (loans, investments, scholarships, etc)
- high tech stuff (electronics, VR/AR, etc)
- industrial equipment also pays good via Adsense (stuff like laser measurement tools for example)
There are way more high paying niches, but I can't think of them at the moment, but if you'll spend a few hours checking various keywords in your favorite SEO tool I'm sure you can find at least 50-60 niches that you can potentially target