SQL AlwaysOn Data Sync
What is the best practice to setup the file share for data synchronization? Network path or local machine 


You have to really read up on AlwaysON. AlwaysON ships only log blocks. So there is no fileshare concept.

On the windows cluster, you can set a witness as fileshare. So, the witness (disk or fileshare) is not AlwaysOn specific. It is required by the Windows Server Failover Cluster to maintain quorum during node failures. Check What exactly is a File Share Witness and when should I use one?

Some good references to get you started :


AlwaysOn Availability Groups Quiz and FAQ [Video] - by Brent Ozar.
Overview of AlwaysOn Availability Groups 
Creation and Configuration of Availability Groups