I am new at blender and want to make realistic models, animations for Unity but could`not figure out which render engine should I work on blender.
Rendering or image synthesis is the automatic process of generating a photorealistic or non-photorealistic image from a 2D or 3D model
This implies generating an Image, which will be a projection of your model to a 2-dimensional space. Anyway, as long as you are not programming a sprite-based game, I don't think what you want to have is an image of your model, but a model to be rendered within the game.
For that purpose rendering in Blender is completely irrelevant and hence is the rendering engine.
Sidenote: If you are asking which rendering engine to use with Blender for photorealistic modeling in general, please drop the Unity from your question.
Actually, in Blender, the Engine that you choose has an impact on work flow as well. I would suggest using the Blender Render engine. In my experience it uses materials and textures more close to what Unity uses. Cycles seems to be designed to only use nodes and I've found that to be far less useful for Unity workflow.