I have different opinion here. One can make blog just for earning as well, but this is not possible for everyone; reason is that blogging requires bunch of abilities in you, such as
1 - Good journalistic and writing skills
2 - Extensive Research Capabilities
3 - You should know PHP
5 - Server Administration (once your traffic goes beyond the limits of shared hosting, you may have to lease and maintain a secure web server)
6 - Little networking
7 - SEO expertise
8 - Good at social networking websites
10 - and above all, you must be a quick learner
If you have these things in your mind, then you can!!!
Just select a topic that interests you most, and start blogging : - )
Any other thoughts?