Well, blogging is easy. You can start right away. What is not so easy is writing fresh articles with unique perspective and content. For you to even think about earning money from blogging, you need to have a good page rank with can only be increased if you write articles in a regular intervals and with no plagiarism.
If you post regularly, build enough readers and pageviews, then in maybe 6 months you can "apply" for adsense.
I recommend you the following links from my personal bookmarks:
How to Blog: Blogging Tips for Beginners : @ProBlogger
and
3 Ways to Earn Money Through Google Adsense - wikiHow