If you want to start earning some extra revenue from being a blogger, you are going to have to have content on your blog that people want to see. Don't just start a blog and slap on some ads, you are going to have to give the visitors material they want to "eat up". Blogs had first started as online journals but now have turned into far more. Now, there are tons of people using blog software to create websites. I know this, because I have done this myself. I am now creating niche sites using Wordpress, one of the most popular blogging software out there. You can see my first niche blog website at http://www.pregoguide.com. When you visit this site, it does not look like a journal, it looks more like an organized website with categories and such.
Where does the money come in? Well that will come in time. My favorite source of income from blogging is from Google's Adsense program. To join Adsense signup for an account and then place code into your blog posts. The code then appears as an ad once posted. When users click the ad, you make money. The price you make per click varies each time but can sometimes be more then just a few bucks. It all depends on the market/niche your blog falls into. See Google's Adsense will pull ads relating to the content on your website.
Another way to make income from a blog is promoting affiliate links. You can signup to relevant affiliate programs, then promote them on your site. You can make a review on an affiliate product, then when the visitor reads your review and clicks the links and buys the product you are in profit. You can also add affiliate links to the menus in your blog, your signature lines, etc. The customer can't buy if they don't have a link to click on.
I started blogging with a personal blog over a year and a half ago, now I have over a dozen blogs and growing. I have actually been trying to create a blog for each website I own. Adding a blog is an easy way to add content to your site. I just installed a blog today on my site SuccessMom.com at http://www.SuccessMom.com/mommy-articles. My blog's content will mainly be articles written by moms or with moms in mind. I plan to publish a few articles daily on this blog of a few different categories. That way I have more chance of the reader finding something they want to read. I am using Wordpress for this blog as well. With Wordpress, you can schedule your posts ahead of time. So today, I setup a weeks worth of article posts.
If you haven't started blogging, what are you waiting for? Not only can you make money with your blog, you can drive traffic to your sites with them. I started with www.Blogger.com. They offer free blogs and you can have one setup within minutes. Try starting out with a personal blog for practice, then go to a business blog. Remember, the more incoming links to your sites, the better.