if you want a ready made website there are some sellers in the marketplace that offer this service. But even if you buy the website, or build it yourself, you will still need domain name and hosting, both of which will cost you anything from $1-$20 a month to $10-100 per year, depending on which hosting company you are signing up with.
Once you have the domain name and hosting all set-up (including the website built) you need to generate traffic which, in my opinion, is the hardest thing to do nowadays in the SEO / make money online industry. And when I say "generate traffic" I don't mean get a few visitors here and there from random sources or social media. That's easy to do. I mean get consistent traffic, passively (or with minimum work anyway) day and night for years to come. To get to this level of traffic generation (oh, the traffic must also be very targeted, too in order for your ad revenue to be good) you will need to either know what you're doing, be consistent with it or, in most cases, both.
Then and only then can you generate ad revenue because otherwise there won't be too many advertisers interested in your 10 targeted visitors a month or 10,000 untargeted visitors a month, or if they are interested then they won't pay you much anyway.
Ad revenue is very good, passive income, but you have to know how to get the targeted traffic, continuously, which is where the trick with making money online is.