Contracts - Solidity Documentation [15 min read]
Begin by reading only the first half of this page in the the Solidity docs, from Creating Contracts up to Fallback Function. Don't worry, we will be returning to the rest shortly!
Expressions and Control Structures - Solidity Documentation [5 min read]
This section of the Solidity docs will show you the many different types of function calls as well as some of the details that relate to this, such as the difference between internal and external calls, how to use input and output parameters, and the scoping of variables.
Function Visibility and Global Variables - Dapps for Beginners [4 min read]
In this tutorial from the Dapps for Beginners series, read the section on function visibility. It is a review on one of the most important topics in Solidity programming.