Roku is my personal choice for getting access to Netflix. The basic model costs only $49 and is extremely easy to set up. It connects wirelessly to the Internet from my cable modem/router. I really like that the remote has something like only 6 buttons on it which are totally intuitive to use.
If you are looking for the quickest, cheapest way to just get Netflix “Watch Instantly” going Roku is your choice. I added an outdoor antenna for live content but another option would be the built-in Hulu Plus.