http://www.wowwiki.com/Instances_by_level is a great start for finding an instance that will work for you. As for being lvl 15, if your horde you can do RFC, if you are boosting with a high lvl go straight to WC. or deadmines if alliance. You can enter the SM instances at lvl 20 and its just crazy amounts of exp if you have a 70 quickly clear cathedral.