"I just got a new Canon Vixia camcorder that records AVCHD video to an MTS file. I want to upload video to YouTube, but it doesn't look like YouTube supports MTS files. How can I convert that MTS AVCHD file to something I can put on YouTube?"
Without knowing a little more about what you want to do, there are a couple of routes you can go to get AVCHD files on YouTube. One option for putting AVCHD on YouTube is to take the unedited MTS file, convert it to anther format, and upload directly to YouTube. The other approach you could take is to edit the video first and then upload the edited file to YouTube. Either way, you're going to need some additional software.
I typically use one of two software apps when I'm working with AVCHD video. Sony Vegas has been part of my video toolkit for a long time, but I'm finding Pinnacle Studio to be more user friendly (and more affordable). The basic version of Pinnacle Studio HD will work, but you might also find some of the features of the in the Ultimate version come in handy if you want to do more complex editing.
Be sure to download and install Pinnacle Studio HD to get started getting your AVCHD video ready for YouTube.
With the software installed, you can then import your AVCHD MTS video files into the application. Drag the video clip you want to convert to the editing timeline. If you need to edit the video before publishing to YouTube, you can make all the edits you want. When you finish making edits, it's time to create the file you are going to send to your YouTube account.
While Pinnacle does offer an option to output video to YouTube directly, you don't want to use it. They automatically convert your video to 640x480. Instead, you want to keep your video at an HD resolution. I recommend using 1280x720 as the format you upload to YouTube.
From the list of options on the Make Movie tab, choose MPEG-4 and HD 720p. Click the Create file button to start the conversion. Depending on how long your movie is, this may take awhile. Keep in mind that YouTube has a time limit of 10 minutes for video.
Using the settings I recommend results in a file optimized for YouTube. When it's finished, sign in to your YouTube account from your browser and upload your video to YouTube.