The Templates Tab:

If you haven’t already picked a nice template from the list, click on the “Pick New” link under the templates tab and select one of the templates that you like and that fits your topic.If you don’t already have an AdSense account, you can click on the “AdSense” link and apply for one straight from the blog instead of having to go to Google.com to apply. That’s how badly Google wants Blogger blog owners to use AdSense on their blogs! :)Once you’ve done that, it’s time to paste your AdSense code into your template. Click on the “Edit Current” link if you’re not already there.There are two methods of setting up your template. You will generate the most clicks by doing the following:
1) Find the section of the template that looks like this:

2) Paste your AdSense code for a 336x280 large rectangle directly after the second

tag. Be sure to format the colors of the AdSense to blend in with the background of your blog for maximum results.
3) Optionally, if you want to show a second 336x280 block at the bottom of the list of posts, look for this code in the template:

Put your AdSense code between the tag and the first

tag.Click the “Preview” button to make sure the AdSense blocks look correct and did not mess up the look of the template. If all is well, click “Save Template Changes”.Note: Once you’ve made changes to your template, if you go and select a different template with the “Pick New” link it will overwrite your changes and you will have to put your AdSense code back into the template.