Within the city itself, the simplest way of getting around is to use a taxi, and the rates need to be agreed with most drivers before you set off, although increasingly, many will have a meter.
Romantics wanting to see the city may like to opt for one of the many horse and carriages (Caleches) that roam through the streets as they have for centuries, and give you a nice high vantage point from which to see the sights.
Although many of Marrakech's attractions and especially the Bazaar need to be walked around, although in the hottest parts of the year, you will end up feeling drained and exhausted because of the sapping heat, so make sure that you carry a bottle of water with you at all times.
If you're planning to head out of the city towards the Atlas Mountains, then you should consider hiring a 4WD car in Marrakech and driving yourself, as this will give you more freedom to see the sights along the way at your own pace.