In C++, you can declare a variable by specifying its name and data type. The syntax for declaring a variable is as follows:

data_type variable_name;

Here, "data_type" is the type of data that the variable will store, such as integer, floating-point number, character, boolean, or a user-defined type, and "variable_name" is the name you choose for the variable.

For example, to declare an integer variable named "myNumber", you would write:

int myNumber;

To declare a floating-point variable named "myFloat", you would write:

float myFloat;

You can also declare multiple variables of the same data type in a single line by separating their names with commas. For example, to declare three integer variables named "x", "y", and "z", you would write:

int x, y, z;

When you declare a variable, it is initialized with a default value, which depends on the variable's data type. For example, integer variables are initialized to 0, floating-point variables are initialized to 0.0, and character variables are initialized to the null character '\0'. However, you can also explicitly initialize a variable when you declare it by using the following syntax:

data_type variable_name = initial_value;

For example, to declare and initialize an integer variable named "myNumber" to the value 42, you would write:

int myNumber = 42;