If you want to add categories to the blogger, read the instructions in this post. For example if you click on category "C#" you will get a list of all the posts that are related to the category C#, and if you click on the category "Blogger Help" you will get all posts related to the Blogger Help category.
The purpose of adding a category is sort of articles by category. Adding a category of your blog becomes easier to browse.
You can write your own HTML to add category links one by one (take a look at topic Creating HTML with category list) or you can use Labels gadget (this is simple method).
Creating a label for posts
First, you need to add a label posts. Label is added by entering text in the textbox "label" which is located at the bottom of the screen for writing or editing posts. For example, I entered the label "Blogger Help" for this post.
Adding Labels gadget to blog
1. Go to Layout.
2. Click on Add Gadget.
3. Select Labels Gadget
4. Enter title, chose to show all labels or only selected labels, sorting type. You can show number of posts per label by clicking check box and display labels as a list or cloud.
I have created a tag cloud for my blog and category list. You can edit later which labels You want to show in yours category list. The end result should look like this.