variable declaration for Oracle in sql developer

Suppose I am in Sql developer and open a new window to perform a sql query. How should I declare a variable? My first line of code below is causing the issue and I don't know why? All I need is to declare a variable on the fly, and then use it. Apparently the Declare statement is not liked by the compiler.


DECLARE MY_NUMBER(10);

SELECT COUNT(1) FROM THE_TABLE;