Marrakech is a fairly sprawling city, and the "newer" parts of town are quite easy to get around by hailing a cab. It's the old walled part of town, the medina where visitors tend to get a little lost. But personally, I don't think that's such a bad thing. There are food stalls everywhere, so you won't starve. There are fascinating little shops and courtyards every square inch, so you'll never be bored. There are palaces and mosques to visit, Riad's to marvel at, artisans to photograph, and fresh orange juice to quench your thirst.
And there's the marvelous Djemma el Fnaa, the main city square, that is unmissable. It's simple: if you get lost just ask for directions to the Djemma.
You Should Get a Guide If...
I would recommend a guide if this is your first time in North Africa. The official guides are very well-qualified historians for the most part, and will no doubt speak your language. They'll help you focus on the details that make this medieval walled city so unique. The historical sights are a lot more interesting when you get the full story behind them.
A guide will also help acclimatize you if you feel a little overwhelmed in the bustle. Guides are also handy to help you ask people for permission to take a photo. In some cases, it's also nice to have a guide help you bargain or let you know what is a "good" deal (but they'll usually side with the seller, rightly so). A half day personal tour is just right to orientate you and make you feel comfortable enough to get lost and do some real exploring later on. Here's a good list of "Things to Do in Marrakech", most of which can easily be accomplished without a guide.
How Much Does a Guide Cost?
If you're on an organized tour, a guide will often come as part of the package. If you're traveling on your own, your hotel/Riad can often recommend a guide they have a relationship with. This is a good idea, because if you're unhappy with the service you have somewhere to go with your complaint. However, you choose your guide, make sure they are a licensed official guide, and qualified to show you the sights. Many official guides are historians and very well educated. They can also speak several languages.
This all helps make the tour more interesting for you. The cost of a half-day private tour will, generally speaking, be around 300 -350 DH, and around 500 - 600 DH for a full day tour. Prices can vary of course, but if you haggle down too much, you may end up spending a lot of time in carpet shops or other spots where the guide gets a commission. Which leads up to...
You'll Still See a Carpet and Perfume Shop ...
Be warned, any tour guide, no matter how private, will take you to a "perfume" shop (disguised as a pharmacy) as well as a carpet shop. It's unavoidable, you just have to go with it. Enjoy it. Accept the cup of tea and don't feel pressure to buy anything. Just give the guy who rolls out a hundred carpets for you to look at, a little tip. If you are really against going to any shop at all, then let your guide know before you start your tour. It may or may not help.
There is a group walking tour of the Marrakech medina that comes highly recommended, but I have not experienced it personally, here are the reviews...
Lost and Scared on Your Own in Marrakech?
If you're lost and feeling harassed by people following you, or asking "where you're from", duck into a shop, museum, restaurant or Riad. Get your breath back, have a cup of tea and ask the owner of the establishment for directions back to a landmark you are familiar with, the "djemma" is an easy one. Do not pay a child to help you find your way. It will only encourage more children to seek this form of employment and may discourage some from going to school. Always ask a shopkeeper instead. They won't leave their shop/stall to take you on a wild goose chase.
Don't ask those who are following you for directions, they'll likely steer you to a shop of their choosing instead. And as much as you may feel intimidated sometimes, don't lose your cool and remember that violent crime against an individual is actually extremely rare in this part of the world. It's a lot of bark, and not a lot of bite.
Most hotels and Riads will have a handy little map for you, and all decent guide books will have one too. You can download maps and walking tours to your phone or i-Pad. Tourist information offices have free maps.