The first thing you need to do is identify it.  The eBay "Historical Memorabilia" board will probably be the best 1st stop.  Photos of both sides are needed (at least 500 dpi) and make sure they are focused.  A scanner is excellent for this.  It'll be ID'd fairly quick.  Make sure you mention any writing anywhere (especially on the edge) that's not part of the die design and include that exactly as it appears.  Be aware that you may receive off-eBay offers to buy the medal with some sort of story attached to the reasoning. 

You do not need to include why the medal was awarded.  Spend your efforts on describing the physical appearance of the medal - all knocks, coloration, scratches, ribbon fraying,  etc.  Look at some of the other medal listings by reputable sellers to get an idea.   Condition is generally not as importantant in medal collecting as it is in coin collecting so don't be overly concerned as long as the photo is accurate.  Do not make any attempt to clean or repair the medal!

Each medal is unique and you need to be aware that variants exist.  Again, a good photo is required - which will probably reveal the info the collector is seeking.  In the case of named medals, the value is based on the recipient.  Be prepared for questions that will be asked and include them in your listing.  With medals, it's sometimes difficult to compare similar medals - you need to compare the exact medal.

You next obvious question is the price.  In medal collecting, eBay generally has the final bid (usually a last-minute bid) as a close approximation of the actual value.  For this reason, I find it's usually quite immaterial as to what your initial price is.  Since you're not an expert, I'd definately avoid listing your medal as a "buy it now".  Medals sold for $$ hundreds have sometimes started out at pennies or a few dollars.  Make sure you include as many payment options as possible because medal collectors are world-wide.   Using a reasonable start price with good shipping terms and methods of payment will probably result in a successful sell.  Please visit my blog (http://blogs.ebay.com/tsa-li) and other guides for further information.