Getting to the Medersa Ben Youssef following a map is almost impossible. I recommend walking along the souqs North of the main square in Marrakech. If you can't find Ben Youseff, ask a local vendor.
The Medersa Ben Youssef is a Koranic school that dates from the 16th century. To enter the Madrasa you have to buy a ticket. The site also offers a combined ticket that also gives you access to the Marrakech Museum and another neighboring temple. I do recommend buying the three entry ticket.
Although the Madrasa is not a big site, you can visit it in less than an hour, it is a must see for anyone visiting Marrakech.