TBH Adsense is the worst way to monetize a blog nowadays. Better just promote affiliate products.
There are guides about finding a niche here or on youtube. You just need some research, no need to pay anything.
If I was starting a blog now, I would focus on a small sub-niche that can be easily monetized, one that can also be expanded later to other sub-niches of the same niche. Preferably an evergreen niche.