If you are using a blogger template of mine, adding a logo to your site header is an easy task (for most of blogger themes). First create your header logo and go to "Layout" page of your blog.
Click on "Edit" link of your Header widget.
A new window will open.
Select "Instead of title and description" as the "Placement" option.
There are 2 ways to add your header logo image.
1. From your computer - using this option you can upload a logo image from your hard disk.
2. From the web - using this option you can give logo image URL to the input field has given. (If you already have uploaded it to somewhere)
I recommend to continue with "From your computer" option, because uploading your logo to blogger, it will load fast.
Select logo image from your hard drive.
When upload complete, it will look like this:
Save Header widget and refresh your site to see result. Final results will be similar to below: