First thing I would do is download a good fan & temperature monitoring app to see what is happening. I like this one [http://www.tunabellysoftware.com/tgpro/|Temperature Gauge Pro], others will work as well. It should help isolating out what is getting hot here.
I would also monitor was processes are running and what load they are putting on your system Check out Activity Monitor in your Applications Utilities folder. Its' possible you have a stuck process or infection running on your system.
While your system is on the newer side its possible the thermal paste on the CPU chip is not doing what it needs to do. Follow this IFIXIT guide to get to the heat sink [guide|12347|MacBook Air 11" Mid 2012 Logic Board Replacement]. Make sure to use a good quality paste. Here's a guide on how to clean the parts and apply the new paste [guide|744|Technique: Applying Thermal Paste].