Before you can begin to apply a value to your comic, you need to understand how to grade them. Comics are graded by condition, on a scale from poor to mint. While very few comics are ever given a true mint grade, a great number can fall into the poor category. The grade directly affects the value of the book and, at times, can be subjective based on who is doing the grading. You can pick up a copy of the Official Overstreet Comic Book Price Guide for a tutorial on how to grade your books. Grading detractors include creases, spine bends, discoloration, tears, stains and fingerprints, among other detriments.
After you have determined a condition grade for your comic book, find out what the value is for the same issues in the same grade. You can compare values online through a number of comic retailers, such as mycomicshop.com, or by swinging by your local comic store and asking the owner or manager what price they put on that issue. You can also look it up in the Overstreet guide or Wizard Magazine. Keep in mind that comic values range somewhat based on geographic locations, as well. A copy of X-Men No.137 might sell for $50 in near-mint condition in Colorado, but bring in $65 in Florida.
If all else fails, you can bring your book to an expert for appraisal. Companies such as Certified Guaranty Company (CGC), Metropolis Collectibles and Professional Grading eXperts apply a certified value to collectible comics. CGC will not only professionally grade and value your comic, but will encase it in a permanent archive package guaranteed to protect your book. If going that route is more than you care to do, call the local comic store and set up a time to speak with a buyer. After seeing your comic up close, buyers can tell you what your book is worth to their stores.