It doesn't; no eMachines comes with a built-in Bluetooth Module.
You'll need to buy one online, go to newegg.com and search for Bluetooth devices you can put in your PC to link up to another device, like you want.
This means you will have to physically install the hardware, and then software, onto the motherboard of the computer, which if you are not well-accustomed to the insides of a computer, I do not recommend you try to do by yourself.
Alternatively, if you are willing to pay, go get it done by a professional.