If you want to make money from your blog, then most free blogging platforms are going to make that difficult. Put simply, in return for providing you with a free blog, they will incorporate their own ads into your site, and you won't get the money for any click-throughs; they will.
The big exception is WordPress.org. Because this is open source software, you will have total control over what happens on your blog, and that includes deciding to feature your own ads, for which the money will go straight to you. However, be aware that WordPress.org does have a big learning curve to use, and you will have to pay for web hosting and a domain name separately.