Dont be worried if you have never sold anything before, just do a lot of research to see which things people will buy, and how much people are willing to buy for.
Take some pictures of your item and put in a good discription including any flaws it may have; be honest.
some people prefer to 'buy it now' and others like to bid for items so its best to have a few of each running at the same time.
Always make sure that you send your item, well wrapped, and very fast. also make contact with your buyer if there are any problems as soon as possible and keep them informed.