Welcome to Shopify! My name is Lisa from the Social Care Team.
Our help center is a great resource for learning the basics of how to edit your theme. First off, here's a sheet of the terminology of the themes so you can understand what parts of the theme the articles are referring to. Here's a great tutorial video to start with, outlining how Shopify themes work.
It's important to note that Shopify uses 'Sectional Themes'. What this means is the theme editor toolbar is divided into Sections that can be dragged and dropped to customize the theme layout how you like. There's also a tab called Theme settings, where you change the fonts, colors, and size of the text. Learn more about sectional themes here and how you can use the dynamic theme editor to customize your site.
All themes that are from the Shopify theme store, whether they're built by Shopify or a third party will have the sectional theme features. If the theme is from a site like Theme Forest, it often does not have the sectional theme features and requires more advanced knowledge of code. After the structure and bare bones of the theme has been built out, the way the themes are customized depends on the theme developer. Shopify offers information on how to edit your theme code here, however lot of these tutorials need to be adjusted to the theme developers style of code.
From what I can see, it looks like your site is using a theme called 'California'. Here are the help docs specific to the California theme made by the developer 'Small Victories'. This will cover general questions about every section of your theme. They also have a contact form at the top right of the page if you have any specific questions about the theme that is not covered in the help docs.
Hopefully, this gives you more insight into your theme on Shopify! If you have any further questions about how to do something on your new store, please let the forum community know here.
Best of luck!