Declaring variables in Visual Basic 2015 and its uses

Hello!
I would like to know what is/are variables. How to declare it in Visual Basic 2015? Can you show me a sample or way how to declare the variables? What is dim? Why do we need to declare local variables and global variables? What is/are differences between local variable and global variables? What are the purpose of it? How useful is it in computer programming? Can you give me ways on declaring different data types using variable declaration?
Thank you!
