In general, baseball cards made prior to the mid-80's have significantly more value. Between the mid-80's to recent times, baseball cards were overproduced, which really squashed the value.
www.beckett.com is a good source. Autographs or game-used jersey / bat / or other memorabilia of players always add more value. As a general rule, the older the card, the higher the value. If you've got cards from the 70s, 60's, 50's or earlier, you might be sitting on a gold mine! Best of luck to you! Overall condition of the card is also important....