There are no real hard-defined rules for starting a blog, but there are tasks you can complete before a full launch that will help you gain footing a bit faster than if it the steps completed after the fact.

If you haven’t yet decided upon a domain name yet start researching domain names by looking into a keyword tool.  A very popular choice is Google AdWords Keyword Tool.  Type in the broadest term for what type of blog you are looking to start and narrow it from there.  For an example, if you sell chairs you may want to research furniture and narrow it down from there (drilling it down to your local area).

Come up with at least 10 post idea and write them (paying some extra attention to the keywords that you found during your earlier research). Even if you may intend to post weekly, monthly, or even daily having this cushion to fall back on may prove exceptionally helpful.  There is always the chance of something happening that you aren’t planning on, and having that cushion will be incredibly helpful.  Another benefit of having some pre-done posts is the ability to take your time and truly craft them perfectly.
Research every angle before writing.  Before you look into your posts, research your market in every area you can find.  There are internet forums for almost anything conceivable, look towards them to get a feel for what your potential customer is looking for. This is also a great time to establish a presence on these forums and work up trust before your launch.

Before you officially launch, you may even want to consult with experts both in your field and SEO and blogging.  You can learn a lot from other people’s knowledge and ability and it will prove a great time to learn from them before an official launch.