I plan to travel to Dubai and i know that they use AED (Dirhams) as curency. I'm from France and I would like to know if it is better to go to Dubai with American dollars or euros? Where should I exchange them? in France or in Dubai?
Your best bet is to come with 100 Euros for emergency use, then withdraw cash from an ATM. You'll get the best exchange rate this way. Credit cards are common in Dubai and you should be able to use them pretty easily for a lot of things (and this is much safer than cash too!).
Also, you can check with your bank. If you have an international bank like HSBC, they might have a branch or ATMs in Dubai.
There are money exchanges in Dubai (many!) but the rates are much worse than if you use an ATM or bank.
There are ATMs all over Dubai airport (and the UAE). The rate that your bank charges you to withdraw - depending on the amount - will be less than your loss for exchanging money.
Example: if your bank charges you €2 to withdraw from a foreign ATM, and you withdraw the equivalent of €200; that is 1% loss; whereas converting to Dirhams in your airport's exchange will likely be a greater loss.
Use the ATM's - they're safe and there are lots to choose from. The lowest penalty I found was ADCB, but that may have changed in the last year.
UPDATE: I think the lowest amount to withdraw from most ATMS is 100AED, which is the equivalent of €15 or 20. So just keep that in mind - it doesn't change my recommendation, but if you just wanted 10AED or whatever, it won't be possible.