Movement.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Movement : MonoBehaviour {

    public Rigidbody2D rb;

    public float speed = 7f;
    public float jumpPower = 300f;

    bool onGround;

    void Start() {

    }

    void Update() {
        float x = Input.GetAxis("Horizontal");
        rb.velocity = new Vector2(speed * x, rb.velocity.y);

        if (x > 0) {
            transform.eulerAngles = new Vector3(0,0,0);
        } else if (x < 0) {
            transform.eulerAngles = new Vector3(0,180,0);
        }

        if (Input.GetButtonDown("Jump") && onGround) {
            rb.AddForce(Vector2.up * jumpPower);
            onGround = false;
        }
    }

    void OnCollisionEnter2D(Collision2D c) {
        onGround = true;
    }
}