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 ComicBookRealm.com's price guide will give you an idea as to what they might be worth.  Please be aware that a Local Comic Book 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 comics individually than by selling them in a group.

To locate your comic(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 "The Amazing Spider-Man", try searching for just "am spid".  Locate the series in question and after clicking on the title you will see a list of issues for that title.  Find your book by the number 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.