Hi, im new with Rhino and i have just one wish. 
Lets say i have a model which is over detailed, like this one.
…and my goal is to cut it (one thing i know to do it in Rhino).
How to make it simple, cos i need just outer line and fill it inside? 
I would suggest using ReduceMesh if your model is already a mesh or use Mesh to convert a surface or solid model to a mesh first… then use ReduceMesh to simplify it.
Ok, do you know how to fill it inside, thanks!
Can you post a model with what you have now? I’m not sure what you mean… meshes are like a skin. If it is a closed skin with no gaps it is considered a closed polygonal mesh. You might be asking about something else though…