You call your own functions as you would call any of the built-in database functions.
SQL>
SQL> CREATE OR REPLACE FUNCTION circle_area (p_radius IN NUMBER) RETURN NUMBER AS
2 v_pi NUMBER := 3.14;
3 v_area NUMBER;
4 BEGIN
5 v_area := v_pi * POWER(p_radius, 2);
6 RETURN v_area;
7 END circle_area;
8 /
Function created.
SQL>
SQL> select circle_area(12) from dual;
CIRCLE_AREA(12)
---------------
452.16