Header and footer adjustments

Warning: I am a newbie to this!
i want to make changes to the header of my site
https://www.thistonybridge.com
and add text under the header bar. where do i go to do this ( specific directions please!
i also want to split the home page up into a series of boxes with different headings and text.
Finally i want to add a footer section with columns and info ( Social, etc). Can anybody help by suggesting how, or pointing me at tutorials on the subject
Many thanks