You are a bit vague with your question, I am assuming you mean watching a film over your wifi connection from your pc.
Firstly you need something on your PC to stream with, I have found serviio extremely easy to use, or PS3mediaserver
You then need an app on your nexus, as others have suggested ES file explorer is extremely easy to use. You also need a player to play it on, a lot use MX player which seems to play most files although I have found that it has issues with some codecs, another one I like is Archos video player, this seems to handle more files(but not AC3 for some reason) and you can also download the metadata for the film which is quite nice