As far as I know, it used to be that the only way to get your videos to play as an ad before other Youtube Videos was through Adwords, Google's advertising platform. You have to set up an account and then link it to your YouTube Channel in order to build your campaign.
While that option still exists, there is another way to do it right from your YouTube Creator Studio. (basically just the admin area).
Assuming that you already have a YouTube Cahannel setup and a video uploaded, Just go to your Creator Studio and then select Video Manager. Find the video you want to promote and click the downward arrow next to the edit button.
This will show you a menu and promote is the very last option. Once you click promote, you have the option of playing “in stream” - that’s before another video on YouTube, or “in-display” which is at the top of the thumbnails on the right.
Obviously in stream will get more views but users can skip the ad so be sure to make those first few seconds count. You can also set your budget and refine your audience by location and interests just like Facebook.
Anyway, that’s it for me. If you want a step by step guide on how to set up your YouTube ad, just YouTube it... I gotta grab another cup of coffee and get my day started.