Variables are initialized by simply assigning a value to them (unless strictvars mode is enabled). It may be useful to initialize a variable to the empty string. A special constant, named nil is used for that.
S = nil // s is set to the empty string
S = "" // same thing
You can also use the var statement to declare a new variable and set it equal to nil. For example:
var s // s is created and set to the empty string