Laracasts already has a course on Shopify development that covers Liquid language basics and advanced topics. You can find it here: https://laracasts.com/series/shopify-for-developers. This course will teach you how to develop and design Shopify stores for clients using Liquid language. Additionally, Shopify has extensive documentation on Liquid language that you can refer to: https://shopify.dev/docs/themes/liquid/reference.
Here's an example of how to output a variable in Liquid:
{% assign name = "John" %}
{{ name }}
This will output "John" on the page.