How to end this script

This is why it's important to properly indent your code. As a beginner, I would NOT put the curly braces on the same line as what's being defined. Put them on the next line so you can easily see which opening and closing braces should line up with each other.

In C#, all code MUST be in a class. You don't have a class defined, so you have to create one.

Your code should look more like:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using ChartboostSDK;

namespace game
{
    public class MyClass
    {
        void Start()
        {
            Chartboost.showInterstitial(CBLocation.HomeScreen);
            Invoke("showInterstitial", 60.0f);
        }
    }
}