First time tax filer here....

When I was 5 yrs old in 1990 my grandmother, who was the custodian of the account FBO myself, purchased shares of a mutual fund. In 2002 she passed away and my mother removed my grandmothers name as the custodian and instated herself as the custodian. Any gains and dividends were reinvested since inception of the account. In 2008 my mother liquidated the account per my request and I took the funds and reinvested at UBS. for the 2008 tax year I received a 1099b from the mutual fund but it only lists the sale date and gross proceeds from the sale. I called the firm handling the fund and they said they don't calc cost basis but they could tell me the date of original purchase quantity and price but that was about it. They maybe can try to dig up old statements.

All that being said, if i file with only the information i was given, then i'm stuck with a huge tax bill. What method do I need to use to arrive at the most effective cost basis so I can minimize my gains? and how do i do it? I haven't a clue where to begin or what to do.

any help appreciated! thanks