Hello there! :))
Sometimes I receive questions about how to get the header to fit into the middle of the blog.
It's pretty simple. Follow the images below, and I think you will understand .... ;)
Now you see Blogger in my Swedish version in the image above.
Even if you have a different language you see where you shall click.
1 -Select 'Mall' (template)
2 -Choose 'Anpassa' (customize)
Now select 'Justera bredder' (Adjust widths)
Here you need to test and try out the correct width.
(The width of the particular background you have chosen).
The width is probably somewhere between 980 px and 1200 px.
Hope I have been able to explain now in an easy way :)
I do my best in English (smiling)