Top Down Character Rotation with mouse

I want to rotate my Player with my mouse and let him look in the direction my cursor is located.
Sadly I am pretty New to Unity and have no clue how to even start…
I am using C# and no charaktercontroller… I Hope you can help me

Have a look at Quaternion.LookRotation():

Out of the top of my head this should be pretty close to what you need:

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public Transform target;
    void Update() {
        Vector3 mousePosition = Input.mousePosition;
        Vector3 targetPosition = Camera.ScreenToWorldPoint(mousePos);
        Vector3 relativePos = targetPosition - transform.position;
        Quaternion rotation = Quaternion.LookRotation(relativePos);
        transform.rotation = rotation;