My name is Laura, and I am here to help you today!
So to add information to your footer please enter your store's admin page. Scroll down to 'Themes' on the left hand side and then you will see 'customize theme' in a blue button on the right. Here you will enter your themes options.
On the right hand of the screen you will find 'footer' and scrolling down through that you will be able to find 'footer contact text' that will allow you to add your contact information.
Depending on your theme would make it clearer as to what options you have for the header. Most themes will not allow for contact information to be added to the header, such as my own store and I am using the 'react' Shopify theme.
Overall, most customers know to look at the footer for contact information so I am sure your orginal plan to add it to the footer will work well.
Let me know if you had any more questions,
Laura's answer was pretty good!
In the case of your theme, Carla, I noticed you have 4 columns of information. One of them would likely have to go, to include anything else. If you wanted a quick "About us" page in there, our theme support team could likely sub out the linklist for an about page - that or you could write a quick "blog post" for down there, and choose the "About" blog, to already utilize what you have!
If you'd like our designers to take a stab at it, you can email firstname.lastname@example.org with your request :)