For the prerequisite steps, such as how to install Unity Advertisement package and integrated into project, please have a look at this:
Unity: Integrate the Advertisement package and monetize your game (Part 1/4)
Install the Advertisement package and setup environmentThere are few things needed before integration, and first of all you need toinstall the Unity Advertisement package: Step 1: Install Unity Advertisement package:Till now, the latest version is 3.4.7, you can use newer version if you like. …

Banner ads is one of the most popular ways to show ads in most of the game. We can create a banner ads by Add placement and then add a new placement:


Now you can create an empty game object and load the banner at game starts:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Advertisements;
public class BannerAdManager : MonoBehaviour {
#if UNITY_IOS
private string gameId = "1234567";
#elif UNITY_ANDROID
private string gameId = "1234568";
#endif
public string placementId = "my_banner";
public bool testMode = true;
void Start () {
Advertisement.Initialize (gameId, testMode);
StartCoroutine (ShowBannerWhenReady ());
}
IEnumerator ShowBannerWhenReady () {
while (!Advertisement.IsReady (placementId)) {
yield return new WaitForSeconds (0.5f);
}
Advertisement.Banner.SetPosition(BannerPosition.BOTTOM_CENTER);
Advertisement.Banner.Show (placementId);
}
}