SET SERVEROUTPUT  ON; -- With this the server communicates with you!
DECLARE
    test VARCHAR2(15); -- delcaration without any data of cvariable test
    test_a VARCHAR2(15) := 'nliplace.com'; --here we declare and initialize the variable test_a
BEGIN
    test_a := 'testvalue';  -- Here we initialize in execution block
    DBMS_OUTPUT.PUT_LINE(test); -- Here we request to the server to print the value of the variable
EXCEPTION
    -- Here you make exceptions
END; -- Here you end everything for this block


-- the output will be "testvalue"