First, if you're trying to find out the value of your cards, you may want to visit your local sports card shop to buy a price guide.  Look in your yellow pages under "Sports Cards", or "Sports Memorabilia", or "Collectibles", then visit the dealer in person.  I recommend not asking them over the phone how much your cards are worth. Dealers receive tons of these calls already.  If you show the dealer your collection in person they can give you an idea on the value of your cards and may even be willing to make an offer.

Second, once you have an idea of what your cards are worth, you may want to consider breaking your collection down.  Selling the valuable cards individually will likely get you more money than selling your collection as a whole.  Selling them individually takes more work, but usually means more money.  It's up to you to decide what's more important, time or money.

Okay, you want to sell them.  Two common options are:

1) Taking them to a local sports card dealer and see what the dealer is willing pay for them.  If you get 50% of book value for your cards, consider yourself extremely lucky.  Getting 5% to 25% of book value is more realistic.  If your cards are old, and by old I mean from 1900 to 1960, you have a better chance of getting close to book value.  Most cards from the late 1980's and early 1990's are extremely plentiful, and hardly any dealers need cards from that era.  If they're actually interested in buying cards from that era, expect to get anywhere from 5% to 25% book value for them.

2) Sell your cards on eBay.  Again, if you sell the valuable cards individually, you'll likely make more money than if you just sell your whole collection.  Be sure to show photos of the cards, and describe the condition of the card(s) as best as you can.  If you have a high feedback rating on eBay, that will help.  If you don't have a high feedback rating, or you're not familiar with eBay, consider asking a friend who is familiar with selling on eBay to help you.

If you decide to sell your collection as a whole on eBay, give a good description of your collection, such as the year and brand of your cards, and the condition the cards are in.  The more pictures, the better.