working on unit testing - what should I be doing here? I made an attempt but feel I am missing the point

namespace TreehouseDefense
{
    public class Map
    {
        public readonly int Width;
        public readonly int Height;

        public Map(int width, int height)
        {
            if(width < 1 || height < 1)
            {
                throw new System.ArgumentOutOfRangeException(
                    "Map must be at least 1x1");
            }

            Width = width;
            Height = height;
        }

        public bool OnMap(Point point)
        {
            return point.X >= 0 && point.X < Width && 
                   point.Y >= 0 && point.Y < Height;
        }
    }
}

using Xunit;

namespace TreehouseDefense.Tests
{
    public class MapTests
    {
        [Fact]
        public void OnMapTest()
        {
            OnMap();
            Assert.True(false, "This test needs an implementation");
            Assert.True(true, "This test is true");

        }
    }
}

using System;

namespace TreehouseDefense
{
    public class Point
    {
        public readonly int X;
        public readonly int Y;

        public Point(int x, int y)
        {
            X = x;
            Y = y;
        }

        public double DistanceTo(Point point)
        {
            return Math.Sqrt(Math.Pow(X - point.X, 2.0) + Math.Pow(Y - point.Y, 2.0));
        }
    }
}