if(select column from table where column = :var_input from table)? wouldn't work :(

I'm sorry but I've no idea how should I use if, end statement from the pl/SQL

this is the code i hope it explains what I want it to do

DECLARE 

        S1 NUMBER(2) := :ID;
        S2 VARCHAR2(32) := :USERNAME;
        S3 VARCHAR(32)  := :PASSWORD;
        X  VARCHAR2(300);
        BEGIN

        INSERT INTO TEST1 (ID,USERNAME,PASSWORD)
        VALUES (S1,S2,S3);
      COMMIT;

    IF((SELECT USERNAME FROM TEST1 WHERE USERNAME = S2) =S2 )
        THEN
        X:= SHOW_ALERT('ALERT12');
         IF (X=ALERT_BUTTON1)
             THEN EXIT_FORM;
         END IF;
    END IF;  

    END;