On your sell your item form (when listing your acution) select 3 types of shipping methods (such as 'priorty,media mail, parcel post, ups, global priorty'). Once you have entered your zipcode on that same form,it will automatically state what the shipping cost will be . If you are just using a flatrate shipping cost, then you will have to weigh the item and then look up what the cost will be according to how much it weighs,,(using the shipping calculator) You can find a quick link for the shipping calculator on the 'sitemap' page. A link to the sitemap will be listed at the top of any ebay page....
You might want to read through some of the selling tutorials that they have available on ebay, they will kind of walk you through the method, including how to figure shipping costs...