You should be aware of the purpose and target readership of the blog before creating it. The purpose may influence the selection of an appropriate blog creation tool, e.g. to what extent are collaboration facilities required. It may also influence the overall style of the blog – a corporate blog is likely to be more formal than one devoted to a rock band.
Target readership is also an important consideration as this is likely to influence the design of the blog. One critical aspect to consider here is readability. Some useful information can be found at: http://www.write101.com/letham2.htm