It starts with having something worth watching. Then it's word of mouth; your friends, then their friends. But that'll only get you so many clicks. You gotta hit more people at one time. If it's a band, that would mean playing out. Eventually, when word of mouth has spread as far as it will go, hopefully, the videos would start promoting themselves. That's where they go viral and all that, if your video is good. (Notice I didn't say song!!)
There are schemes these days where you pay for clicks. But again, that'll only get you so far, and you have to pay for it.
There are many ways to promote your YouTube video --
1. You can add your video in content (you can post it in article or web2.0 sites)
2. You can share your YouTube video on other video sharing websites
3. Promote it on your company blog.
4. You can share it social media sites
5. Share your latest video with your YouTube community