The mutual fund itself should be sending you statements showing the shares you owned and any cost basis they have for you. If they don't have anything for you, you'll have to do the best you can to estimate a cost or else enter zero and pay tax on the full amount of proceeds from the sale.
To estimate the cost can be pretty difficult, especially if it had dividends reinvested over the years.
You would have to determine when you think the original purchase was made and see if you can find a price for the fund around that time by searching for historical prices online. If you have shares that came form dividend reinvestment, you could cost those shares at the average price of the fund during the years the shares were held.
If this is a small amount of money, it may not be worth your time to do anything but use a zero.
If your mother could help you , that would be convenient but not always possible. Maybe, though, your mother kept some papers showing when she bought the fund.
