Starting out with a credit at 16?

Ok I'm 16, started working since i was 15 as a waiter at my sister's Restaurant. I have my own savings account and i need to purchase a laptop thats approx $$3000 which everyone in my house is OK with but i want to pay for it on my own. I have a debit card but i dont want to pay for the entire laptop all at once I want to use credit to pay for it is there any way i can start having credit and purchase the laptop and pay like a monthly payment. My sister said i can start off my credit as having her as my co-signer but i still confused on how it works i dont want to keep on asking her about this. I tried calling my bank person and he just said that i have to be 18 to start my credit. Can anyone help explain how the credit thing work or is there a way that i can still purchase the IBM X60 laptop i want to get.