PlayerMovement.cs

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

public class PlayerMovement : MonoBehaviour {

    CharacterController controller;

    public float moveSpeed = 12f;
    public float gravity = 9.81f;
    public float jumpHeight = 3f;

    Vector3 velocity;
    bool isGrounded;

    public Transform groundCheck;
    public float groundDistance = 0.4f;
    public LayerMask groundMask;

    void Start() {
        controller = GetComponent<CharacterController>();
    }

    void Update() {
        isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);

        if (isGrounded && velocity.y < 0)
            velocity.y = -2f;

        float x = Input.GetAxis("Horizontal");
        float z = Input.GetAxis("Vertical");

        Vector3 move = transform.right * x + transform.forward * z;

        controller.Move(move * moveSpeed * Time.deltaTime);

        if (Input.GetButtonDown("Jump") && isGrounded) {
            velocity.y = Mathf.Sqrt(jumpHeight * 2f * gravity);
        }

        velocity.y -= gravity * Time.deltaTime;
        controller.Move(velocity * Time.deltaTime);
    }
}