c# don't understand calling methods. object orientated programming : I've just started object orientated programming and i don't quite understand how to call another method. I wanted to calculate the percentage increase and then put out the resulting overall price. This would be done by calling the method to do the working out then returning the value to the other method. However i'm unsure about how to cross over between methods. 
Could someone please explain how i'm meant to do this. Preferably not just giving the answer so i can fully understand what's happening. 

p.s. this piece of code is in a class called RetailPricing. As i've copied and pasted it, it doesn't look like it's all formatted properly. (i understand how to call this class to the main program)

Thank you in advance! 


namespace week7exercise2
{
    class RetailPricing
    {

        public void CalculateRetailPrice()
        {
            double inputcost;
            double inputpercent;
            string inputitemcost;
            string inputmarkup;

            Console.Write("Please Input The Cost Of The Item: ");
            inputitemcost = Console.ReadLine();
            inputcost = double.Parse(inputitemcost);

            Console.Write("Please Input The Markup Percentage: ");
            inputmarkup = Console.ReadLine();
            inputpercent = double.Parse(inputmarkup);

            Console.Write("Your Retail Price Is: " + newprice);
        }

        public double sum(double MarkUpPercentage, double overallprice, double newprice)
        {            
            MarkUpPercentage = inputpercent + 100;
            overallprice = MarkUpPercentage / 100;
            newprice = inputcost * overallprice;
            return newprice;
        }
    }   
}