BUSINESS DATABASE

I own a wholesale business in Kingston,Jamaica and i want a personalized program for my business where i can input my business invoices ... in it i want a form to include details , unit cost , total cost , and i want it set up so that it has a button so that when clicked it automatically calculates discounts for particular persons and adds tax ... it should also be able to include statistics such as total sales for month .. and be able to calculate statistics like percetange increase in sales over a specific time period etc .. Project need not look like exceptionally, but as long as it functions like a champion i will be happy with that .