Reducing vert/triangle/whatever else count before exporting into Unity? Question is basically in the title, what should I do to remove everything what basically can't be seen and etc without reducing quality of the outside of the mesh - what will be seen? 

Do I have to be smart and make mesh "optimized" on my own right from the start or can I reduce those counts in some ways in case I didn't think about it at all(especially when I don't even know what in meshes causes lag)?
In my experience you should not pay too much account on the number of vertices  in a mesh. What really slows down unity is the number of draw calls caused by using too many materials in a scene. 

When you have more than 10K vertices of an object you might want to consider creating less detailed objects and use them with level of detail.