If you do not want to spend any money outright start with a blogger blog (free) and learn topromote it through free traffic generation tactics. Once you earn a little bit of money through your free blogstart with a WordPress blog (free again but you need to buy webspace and domain name).
Decision No 1 : Blogger blog or WordPress blog
Decision No 2: Choosing a Niche Topic
Decision No 3: Want to create the blog yourself or get help?
Decision No 4: Do it yourself? Then which theme? Get Help? Then where do youwant to go for help?
Decision No 5: Content development. Do you want to write your own content? BuyPLR? Get free content from article directories?
Decision No 6: Promotional tactics. How do you plan to get traffic to yourblog?
Think of one topic that you are passionate about. You should be interested enough in this topicto pursue it for many years on a regular every day basis. It may be your hobby, your interest, expertise or whatyou want to learn more about.
Then do some keyword research through Word Tracker and Google Trends and try to find out if there are enough people searching for the topic of your interest.