How to do auto numbering on INT column

HI
I have a simple table created using PGAdmin III. How do i do a auto
numbering on a column SYMBOL_ID?
My table is
CREATE TABLE "SYMBOL"
(
  "SYMBOL_ID" int4 NOT NULL,
  "SYMBOL2EXCHANGE" int2 NOT NULL,
  "SYMBOL_ALIAS" text[],
  "RELATED_SYMBOLS_OTHER_EXCHANGES" int8[],
  "SYMBOL_NAME" text,
  "COMPANY_NAME" text,
  "SYMBOL2SECTOR" int2,
  "SYMBOL2INDUSTRY" int4,
  "STOCK_SUMMARY" text
)

Thanks

Seede