First thing you need to do if you want to start you Blog on Joomla is to create the "Infrastructure" for you Blog Posts.

This means that you have to create the structure in Joomla that will give you the right structure so you can easily work you post into your Blog.

1. Create a seperate Joomla Section that will hold the categories for your Blog, you will need this later on if you are going to look at comment components.
Do NOT call it "Blog", but rather something like "Topic Blog".

Reason for not naming the section and category it "Blog" is simply the fact that some SEF components use the "Blog" name in the URL for a Blog based menu link...

2. Create the categories that will hold your blog topics, here is the same restriction not to name it "Blog".

Just create as many categories as you need to structure the Blog topics you want to write about.

As an example, you want to blog about Joomla! than you can create categories like "Components", "Templates", "Joomla News", "Modules" and "Plugins".