Note: This article assumes you are using a classic template. For Layouts-enabled blogs, you can simply add a new AdSense page element to your sidebar, or drag an existing element to the desired location.
If you're new to AdSense and not too familiar with making modifications to your blog's template, you can follow these instructions for placing AdSense code in your blog's sidebar (using the Minima template as an example).
Assuming you have already signed up and been approved for a Google AdSense account, you should be able to log in here. Once you're signed in, click the AdSense Settings tab near the top of the page. To cut right to the chase, focus on these steps:

1. Choose Your Ad Layout - To fit ads in your sidebar, it's best to choose one of the following ad layouts.

   Button (125x125)
   Skyscraper (120x600)
   Wide Skyscraper (160 x 600) 
   Vertical Banner (120 x 240)
   Small Rectangle (180x150)

2.  Choose a color palette - They are very cleverly named but it's really just a matter of your preference.
3. Copy your AdSense code - At the bottom of the page, there's a box with code in it. Highlight all of that code and copy it.
4. Paste the code somewhere - Notepad or Text Edit or something like that is a good place for now.
5. Go to Blogger.com and sign in - Choose the blog you want to put ads on and click the template tab.
6. This is less of a step and more of a precaution - Copy your entire Blogger template and save it in Notepad or Text Edit just in case.
7. Find the place in your template that says this; you might have to scroll down pretty far:

8. Paste the AdSense code you saved earlier right after that part.

   
   
   PUT YOUR ADSENSE CODE HERE!

9. Save your template changes and republish your blog.