How do I reduce the poly count of an existing 3D model? I have a question about 3d modeling.

I have ZERO experience in 3d imaging, modeling....none!

However, what I'd like to know (if possible) is how difficult is it to take a pre-existing 3d model and reduce the poly count? 

I'm using a little Steam game called Table Top Simulator and it can allow only certain size of OBJ files. Most 3d models I see online are quite large and I'd like to reduce those images for personal use.

http://steamcommunity.com/sharedfiles/filedetails/?id=692037907&amp;tscn=1464716182

Hopefully the link here shows exactly what I'm trying to accomplish. This person has themselves found online 3d models from various sites and reduced the poly count using 3D Studio Max. 

I've heard Blender is more user friendly?

Anyways, if someone can push me in the right direction on how to reduced existing 3d models I would greatly appreciate it. I know this isn't a simple "follow this one tutorial and you're done", but I'd like to know what I'm getting myself into before I jump in (this is for personal game use only and nothing something I'm devoting a lot of time to....but, I do find the urge to learn much!)

Thanks to anyone that can help.
Another way to reduce vertex count is by sculpting out the excessive geometry.
note the low strength and the detailing mode set to collapse edges.  



result :