I have the same TV and it can stream Amazon Video on Demand, Netflix and Hulu by itself. Once the TV is connected to the internet either by wired connection or the wifi adapter you only have to press the internet button on the remote. A menu will pop up on the bottom of the screen and you can pick the service you want to use. I've used mine for Amazon VOD and Netflix and it works great.
Answer here...No you don't need a roku box. You will need to hook the TV up to your internet connection either through ethernet cable or buy the optional wireless adapter. Once that is set up just go to the internet apps and select the amazon service. It will give you a link to a webpage where you will need to enter the PIN it provides. Once your tv is activated on the webpage using this pin you are good to go.
A Roku box is unnecessary, according to my relative who received this TV from me as gift. last year we gave him the Roku box for use with his old set, so I believe he knows from personal experience. Can't help with the how to part. Sorry.