AEM is an enterprise level software and there is no free trail.
You need to contact Adobe sales for information about how to get AEM
Once you have a license, you can follow the documentation for download links at How to download the software for Adobe Experience Manager (AEM) 6.3
It is recommended to attend Adobe Trainings to kickstart the learning process. A trial license will be shared in the training.
You will get a 90 days trial register at http://www.adobe.com/devnet/aedp.html
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