How to load different excel files?

I am currently trying to test the CRM system for our company and we would like to load historic data into the database. I have customers who have already purchased our products, customers who are going to have a demop and leads. I would assume that customers who have already purchased the product would directly go into the 'Potential' category however will i need to creat a contact first? I am just having problems loading previous existing customers into the database without having to load it into the leads section and having to go through them one by one.