[url http://forums.oracle.com/forums/forum.jspa?forumID=75]SQL and PL/SQL forum would be more appropriate. However before placing question there I suggest you to [url http://tahiti.oracle.com/]read documentation, it is quite basic question covered there.
Go to documentation link provided above. Pick version. Write to_char into the search field. Press search button. Read about this function in reference guide (should be the first or one of the first results). You'll find a link to number format elements there.