You can try to sell your song by yourself, or you can have lots of people selling it for you.
If you try to do it alone, know that there are thousands of people just like you. Record companies don't even bother to listen to the zillion demos they get. You need a way to stand out.
Now, imagine if a record company executive noticed a song of yours kept coming up on social media? Imagine if a number of her friends mentioned this song, or "liked" it or somehow acknowledged it? Would the exec have a listen? Surely. This is the new music marketing power method.
Or, imagine that this executive has relationships with agents, and knows that one particular agent always sends her great stuff. Would she open the email or play the CD? You bet. Let a connected person sell my music.
So choose the targets that will get your song heard by the most, and most influential, people. Don't send anonymous demos. Get your song viral on YouTube, or get that super agent to represent you. If you prove that your music speaks to lots of people, and that lots of people will buy a spanking new album from you, record companies will be interested. They are there to make money, not do acts of kindness to starving musicians.
Obviously, you need a killer professional recording of your song. Don't leave home without it. You will also need a killer professional music video. Hire a producer, spend the money. Make it look like it belongs on MTV, or in Carnegie Hall, whatever your ultimate goal is. Music marketing is about making people know, be blown away by and fall in love with your music. the rest will flow easily (so to speak).
Again, check out the Music Promotion Zone for help on getting all this stuff ready, including videos, press releases, a website, social media marketing and more. It's there to help you succeed, so take advantage of it. Oh, and it's gratis.
The greatest tool is persistence. Don't be a stalker, but don't ever give up on your dream. If it's not working, make adjustments. Follow up with all your promotional work on a long-term basis. You can't just post a song on YouTube and wait for the billions of views that will make you the next phenomenon. You've got to reach out all the time. Tweet, share, network. ASK people to refer people. If your music is that good, it will soon take off on its own. Music marketing is a persistent activity.
Then you are ready to go for that agent, that record company.
And if it's not happening, do not despair. Write another song, make another video. Keep going. You know, an executive may not listen to your music the first three years, but by then your name will be becoming familiar. Studies show that people who know only a name of someone still feel some connection. That is huge, so make them get to know you. Make them familiar with you as a professional, persistent musician, not as a stalker.
Keep at it. Be a salesperson - be happy with each "no", because it's one less to go on the road to the big "YES".