Control your camera to look around using mouse

MouseLookAround.cs

using UnityEngine;

public class MouseLookAround : MonoBehaviour
{
    float rotationX = 0f;
    float rotationY = 0f;

    public float sensitivity = 15f;

    void Update()
    {
        rotationY += Input.GetAxis("Mouse X") * sensitivity;
        rotationX += Input.GetAxis("Mouse Y") * -1 * sensitivity;
        transform.localEulerAngles = new Vector3(rotationX,rotationY,0);
    }
}

Improved Script(Smooth Rotation & Sensitivity Axis Control)

using UnityEngine;

public class MouseLookAround : MonoBehaviour
{
    float rotationX = 0f;
    float rotationY = 0f;

    public Vector2 sensitivity = Vector2.one * 360f;

    void Update()
    {
        rotationY += Input.GetAxis("Mouse X") * Time.deltaTime * sensitivity.x;
        rotationX += Input.GetAxis("Mouse Y") * Time.deltaTime * -1 * sensitivity.y;
        transform.localEulerAngles = new Vector3(rotationX, rotationY, 0);
    }
}