First you'll need a YouTube account.
Then you need some kind of video camera (I just use the built-in camera on my Mac, but I'm saving up to buy a good one).
Next, record yourself talking to the camera about whatever you want to talk about then use an editing program (if you want) to put together the best parts of what you said (I tend to have several minutes worth of footage that is just me staring at the camera).
Note: If you use Windows, you can download Windows Movie Maker for free, or if you have a Mac it should already have iMovie in your applications.
Once you've edited you need to export the video (this changes it from a project that can be edited to a ready-to-upload video).
Finally, log on to YouTube and select the "upload" option. Upload the video you just exported, give it a spiffy title, a description, and some tags... and voila! You've just vlogged!
I would suggest watching some other people's vlogs to get an idea of how a well-edited vlog looks. Here are some of the more famous vloggers that I love to watch:
And, of course, you can check out MY vlogs!
Hope this is useful! Let me know in the comments if you have any questions and I'll be happy to answer them.
Until next time, best wishes!