This is an age old question, but the long story short is that they are only worth what someone is willing to pay for them.  Using NonSportRealm.com's price guide will give you an idea as to what they might be worth.  Please be aware that a Local Card Store will offer pennies on the dollar (as they have to resell to make a profit), and places like eBay will generally see a higher margin.

You can also generally earn more by selling cards individually than by selling them in a group.

To locate your card(s) use the search feature at the top of the page.  When searching for a title search for a portion of the title.  For instance if your title is "Journey to the Center of the Earth", try searching for just "jo cen ea".  Locate the series in question and after clicking on the title you will see a list of cards for that title.  Find your card by the number and set and either add to your collection, or just use the price as a reference.

Please be aware that we get a lot of questions about this issue and this is just a canned response.  If you would like more help please use the forums and a member will be glad to help you.