In this module you can learn what a variables and expressions are and how they are used in algebra.
The video above contains an explanation of variables.
The above video from Khan academy explains variables, expressions and equations.
There are no rules that determine what letter is used as a variable.
Variables always represent some value, even if that value is zero.
The number that comes just after "x" is not "y"; it is "x + 1".
Use these search terms to find your own resources:
what is a variable
algebra variables
working with variables