C# Static, Public, Private

I have some experience in VB.NET.  I'd like to learn C# - but there are some concepts that I can't quite get my head around.  The current issue is Static vs Public vs Private when dealing with functions.

Take the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Variables
{
    class Program
    {
        static void Main(string[] args)
        {
            int number = 24;
            AddFive(ref number);
            Console.WriteLine("This is the number...: {0}", number);

            Console.ReadLine();
        }

        public void AddFive(ref int theNum)
        {
            theNum += 5;
        }


    }
}

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The error I get is "An object reference is required for the non-static field, method or property 'Variables.Program.AddFive(ref int)"

So I need help understanding this error - how do I do an object reference from inside itself?
If AddFive is a method of Program, perhaps me.AddFive would work?  (it doesn't)

It seems that Public and Private are used inside instantiated classes, while static is in the main class - but for some reason (maybe it's my procedural upbringing), it doesn't seem right.

I know that changing "public void AddFive" to "static void AddFive" will fix the error - but I don't understand why.
I understand public and private (for the most part) from working with VB.NET - but static, not so much.  

In my main class (Program), I've learned I have to have "static void Main(string[] args)" as an entry point to my program - so does that mean that ANY and ALL functions in my main class also have to be static?

Any help you can provide on the subject is appreciated.  I did do a search on Google - but the responses I saw were basically "that's just the way it is" - and that's not super helpful.

Thank you for your time!

-Steve