If there are no parameters to pass, you can simply call the function without the parentheses.
To pass actual values, you can use commas as placeholders for parameters.
SQL>
SQL> CREATE OR REPLACE FUNCTION squareme(thenum number)
2 RETURN NUMBER IS
3 BEGIN
4 RETURN thenum * thenum;
5 END squareme;
6 /
Function created.
SQL>
SQL> BEGIN
2 DBMS_OUTPUT.PUT_LINE('9 squared is ' || squareme(9) );
3 END;
4 /
9 squared is 81
PL/SQL procedure successfully completed.
SQL>