[support] register form add first name and last name

Hi , i have inserted the new field in the register form , say name is custometxt , This field display in the register form , Next i created the new table in the DB , table name is tbl_user_extra_info , AI id, uid(username uid from usertable), custometxt field as varchar , i know there is hook_insert function there, so i do the insert operation also, when i have to call this hook_insert , bcoz the default insert operation is in the user module, so how to get the that id and how to peform the second table insert operation , Any suggestion for doubts,