Before you start anything: Did you use the offer of Google Account for storing all your passwords,etc?
Are you a beginner or an Android Crack? If you are a beginner, then it is very, very complicated - get asap to a data recovery firm (very expensive) or get to the National Security Agency :)). As Android Crack I would try ASAP (hope you've rooted the device by a Custom-Bootloader) mirror the whole smartphone (on a phone shell apk with sudo/su then dd if=/dev/mmcblk0 of=/ disk with same mem size of smartphone mem) as well as the SD disk to a PC disk, then try to use a Linux recovery disk with gpart, disktype, testdisk, and e.g. aeskeyfind etc. to recover the data. So far I know the keys are in a certain folder (look on the web/stackoverflow) in the system partition, but if you overwrote the phone with Odin/Heimdall (DO NEVER THAT NOW) then you can forget all data.
The next time after you've rooted/ made a firmware reset and new encryption the first thing you should do is to backup the key stuff on a new Memory card and duplicate the keyfiles as much as you can!
Afterwards everything is hell :)) (Even there are APK which are offering a recovery/decrypting of private encrypted files, I would be very cautious of that - they're mostly fakes/adware and maybe trojans)