this isn't an easy question to answer.
First of all it's going to depend on where your blog is hosted. Some sites like Myspace.com doesn't allow you to have adsense ads on your MySpace pages - including your MySpace blog.
Now if you're talking about a blog hosting site such as Blogger.com there are a couple of answers. First of all its going to depend on whether you're using Blogger's old or new format. In their older format you are going to have to copy and paste the adsense coding in yourself and
WHERE you post the coding is going to depend on
WHERE you want your Adsense ads to appear - meaning do you want your ads to appear on the side of your blog or do you want them to appear after your blog posts? This is going to require that you have at least some knowledge of html coding so that you will know where you can paste in the ads so that it doesn't ruin the template of your blog.
However if you are using Blogger.com's newer format then while you can still copy and paste in the coding yourself you also have the option of letting Blogger.com add the codes for you -
However you must first give Blogger.com permission to add the coding.
Once you have done that then you can let Blogger.com add your Adsense coding to either the side of your blog - where you add it as an additional gadget. Or you can add the ads right under your blog posts - of which you are only allowed to add a certain number of Adsense ad blocks.