I think one of the best ways to learn is to just do it. Pick one item to sell at a time until you've got the hang of it.
Look at other peoples auctions, see how they word them, not only the item description but the payment and return policies. I had to play around with my wording over a couple differant auctions to find out what I wanted to say and what I wanted my policies to be.
If you don't have one, you will want to get a digital camera. Posting even a single picture will help SO much in getting your items sold. Most people don't buy things sight unseen, after all! And I second the idea of getting a scale to weigh your packages. It doesn't have to be a postage scale, either. I use a digital food scale that I had bought when I was on Weight Watchers. Works for anything up to 5 pounds. I just take the weight and use the Shiping Calculator on the listing, and then the buyer just pays the actual shiping cost. I've never under-charged for shiping! I also ONLY ship with Priorty Mail, you get free shiping suplies, it's fast, and it's not that much more than parcel post anyway.