You'll get about one click on an ad for every fifty to a hundred visitors. Each ad click will get you 20 or 30 cents. That comes out to .2 cents per visitor. In order to make a dollar a day, you'd need five hundred people every day.
One dollar a day is 30 dollars a month.
If you want to replace your income, you'll need at least 5K per month. That means you'd need over fifty thousand visitors a day to make 60K a year from ad revenue for one site.
To get a first hand idea of how incredibly difficult that is, head on over to wordpress.com, and set up your own free blog. Make a couple of posts, and then come back in a couple of weeks to check your stats. You'll be lucky to have gotten 20 visitors in that time.
Most people that make a living from ad revenue have spent at least a year building several sites, many times over a hundred, so after a year or two, they've got enough traffic to all their sites to replace their day job income.
It's not that difficult, but when most people think of online income, they think of overnight riches, and not putting a year or two of consistent effort, day in and day out, and they give up pretty quickly.
But at last count, there were over a million people in the US making a living through blogging. They just didn't get there overnight.
It's possible, and simple, just not in the time frame that most people are willing to put up with.
There are several "free" places to build sites which will allow you to "share" revenue, and that may be a good place to start, as it's usually easier, but it will take much much longer that way, as you have to share a portion of your ad revenu with them.
Putting up a decent squidoo page, or most other web 2.0 sites is a good way to do that. At least to get started, and get your feet wet, so to speak.
The problem with free sites, is you are never in control, and they could shut down, sell the site to somebody else, change their ad policy. So once you get the experience of what it takes to make some money, it's best to buy your own domain, and put up your own site/forum etc.