The walls run for about 2km in total around the old city. There are several different places where you can get access to the wall (I remember signposting indicating where to go). One place is at the end of the main shopping street - if you are tendered ashore, you arrive at one end of the shopping street and the access to the wall is at the other end of this interesting street (about a 5 minute walk). You can buy a ticket as you enter the wall. The cost 2 years ago was a couple of Euros (even though they don't officially use the Euro you can use it in Dubrovnik). You can get off the wall at any of the several entry points. Best part is along the sea front.
Enjoy it - it's a great experience.