how do i stream videos to chromecast from a pc?

There's a built-in option within the Chrome browser to allow you to mirror your browser OR your computer screen completely to a selected Chromecast. Please be aware that this feature isn't fully supported, so expect some performance issues which includes (but not limited to), A/V latency (between PC & TV), video quality issues, frame drops & periodic disconnection / crashes.

For videos going through Chrome will provide the broadcast to Chromecast where possible.

Just a little more research into the feature should assist you.

Within Chrome when playing a video you should see a "chromecast" icon overlayed on any video you play. Just click on that and it should start playing the video on the chormecast!