Mel has three main types of variables. They are float, int, and string.
(There is also vector, but it's not important yet and can be thought of as a group of floats anyway.)
A float, or floating point, can hold any real number: 10, 13, 17.519, -81.912, etc.
An int is an integer. It can only hold numbers without decimal places:10, 13, 17, -81, etc.
A string holds text. They can be numbers or letters, but they are thought of as text: This string is not 81 characters long.

To declare a variable simply write the type of variable you want followed by what you want to name it:

string $sweetString;

You can also tell the string what you want it to contain:

string $sweetString = "This string is really sweet."

You create numeric variables the same way, except you don't need quotation marks.

int $twelve = 32;
float $number = 156.891

You can perform any mathematical operations on the numeric types.

int $twelve = 12;
int $one = 1;
int $answer = $twelve + $one;
print $answer;


You can add strings too, thats the only "mathematical" operation you can do with them though.

string $twelve = "twelve";
string $one = "one";
string $answer = $twelve + $one;
print $answer;


If you got an error from that second script, read this page about scope.

By the way, you can use the print command to see the value of a variable anywhere in a script.