Help with First Custom Computer Build

So I've never built my own computer before and need quite a bit of help I have around $400
So the kind of computer I want is one that runs top notch games like Skyrim, MW3, etc. well with at least Medium Graphics.
I will also be streaming on twitch and/or post videos to youtube so keep that in mind also.
I will most likely be using it for many hours in the day so equipment I need to have a long lasting computer would be nice.
I would prefer windows 7 unless someone suggests something else for a good purpose.
And feel free to suggest any other hardware/software that will assist me with what I need.
Please and Thank You

-Sincerely, Deividdo