How to add friction to a Wheel of Fortune in Unity3D?

First of all I am new to unity as will as C#, I want to create a wheelof fortune for my game using C# where you could rotate it with mouse.I wrote this code in C# it works good but the only problem is I don’t know how to add friction to it so that if I release the mouse button it should still continue rotating and slow down until it fully stopped (Just like a realistic Fortune Wheel).

using UnityEngine;
using System.Collections;

public class RotateCSharp : MonoBehaviour
    public int speed;
    public float lerpSpeed;
    float xDeg;
    float yDeg;
    Quaternion fromRotation;
    Quaternion toRotation;

    void Update()
        if (Input.GetMouseButton(0))
            xDeg -= Input.GetAxis("Mouse X") * speed;
            fromRotation = transform.rotation;
            toRotation = Quaternion.Euler (yDeg,xDeg,0);
            transform.rotation = Quaternion.Lerp(fromRotation, toRotation, Time.deltaTime * lerpSpeed);

any help will be much appreciated?

Friction is a physics property. Rather than just changing the transform of the wheel, add a RigidBody and then use Unity - Scripting API: Rigidbody.AddTorque to spin the wheel instead. Adjusting the AngularDrag of the RigidBody will change the amount of friction the wheel exhibits.