Basically you first have to create your own blog or website
It's easy, there are numerous websites that offer free blogging and as for the website there loads are free web hosting services out there... Just pick one you want..
My choice is Blogspot.com since it's easier and a lot more user friendly
Alright here's a list of useful ad based income...
1.) Google AdSense
This is the most popular choice. You can create an account on AdSense and have acces to loads of text, image and even video ads. You can actually post these adds on your blog or web page and earn money on a per-click basis. The catch is you have to earn 100$ before you get paid. Not sure if Google decides to lower this pay bracket.
This is similar to Google Adsense however the lower pay bracket of 10$. Well personally I can wait to earn 100$ but for some bloggers, 100$ takes too long and so they prefer Bidvertiser.
Google is kinda popular so i wont be surprised if there are more ads to choose from.
I'll be updating this soon...