You need the Ubuntu driver for the wireless device inside (or outside, if you plug it in the Ethernet or USB port of) your emachine. Assuming there's no wireless device in it, you'll need a Linux compatible plugin thing.
Although it's 2 years old, http://www.cyberciti.biz/tips/linux-usb-wireless-compatibility-adapter-list.html might be a useful start.
But wired definitely is easier. That works out of the box.
I just bought the Clear wifi hub for my home with the roku xs. My emachine T6412 desktop doesn't have wifi capabilities, netgear N300 and Belkin N150 wireless usb drivers don't work with my Ubuntu. Couldn't get any help from Geeksquad at Bestbuy. How do I upgrade to wifi for my pc?