Hi Sophie,
Tricky to explain things easily at a technical level if you've never used Moodle before. Ok, I suggest that you install your own Moodle server locally - what machine do you have? Windows? Mac? Then read 'Installation_quick_guide', 'Admin_quick_guide', 'Teacher_quick_guide' and finally 'User_quick_guide'. Then have a practice on your local Moodle, then read 'https://docs.moodle.org/dev/Themes' so that things make sense.
Kind regards,
Gareth