If your music is 100% your own work, you automatically own the copyright. In this case, you most likely don't need a license. 🙌
Note: The legal situation can differ depending on the country. 🌍
However, if you're releasing a cover, a remix or a song with samples, you'll probably need a license beforehand. Otherwise, you risk legal action from the original rights holders.
You can find more on this in the article "Am I allowed to release covers, remixes or songs with samples?".