SIte counters allow you to get an idea of how many people are visiting your blog and where they are coming from.
There are a lot of different site counters you can use, you will be adding a site counter from sitemeter to your blog.
Visit Sitemeter and sign up for a counter to put on your blog
You will then need to add the code to your blog.
To add to a blog follow these instructions. Instructions

These are the instructions for adding a Site Meter web counter and tracker to your Blog.
  1. Select the HTML code in the box with your mouse.
  2. Copy it to the clipboard.(Press the CTRL key and the letter C at the same time.)
  3. Login to Blogger.
  4. From the dashboard, click on the Layout link for your blog (the link may also say Template on some dashboards).
  5. On the "Add and Arrange Page Elements" page, click on the Add a Page Element link.
  6. On the "Choose a New Page Element" window that pops up, click on the ADD TO BLOG link for an HTML/Javascript element.
  7. Paste the Site Meter HTML in the "Content" section of the Configure HTML/Javascript popup window. Entering a "title" is optional.
  8. Press the Save Changes button to add Site Meter to your blog.
  9. Press the View Blog button to see Site Meter on your blog.