Mixing and Mastering are essential elements in recorded music. Ensuring that all tracks are suitably layered, creates the overall experience. You don't want your song to sound too muddy or empty, so your fans can truly listen to the sound how it was meant to be heard. There are many ways you can make sure you're song is mixed and mastered and ready to be released.
Do It Yourself
A DIY project isn't impossible and may be the only option if you're on a budget. There are plenty of online resources to help you learn, especially if audio engineering is something you'd want to go into longterm. For me, along with being an artist and performer, I want to be an engineer. Just knowing I'll be able to tell you what every one of those small buttons and knobs in a studio do is exciting. If you are looking to learn how to DIY, my two recommendations are YouTube University or Classes on Coursera.
Coursera
Coursera is a site that collects online video classes from colleges and universities all over the world. Many of them are free to take. In my experience, the only time I was asked to pay is at the end of the course. This is just for a certificate of completion. Since I'm learning for my music, I haven't needed them. If I want to get them later, it is easy to do. Coursera provides classes on thousands of topics. Click below to see one of their production programs.
https://www.coursera.org/learn/technology-of-music-production
YouTube University
This is not the first time I have gone to YouTube University to learn something I needed. I've used it to learn and study for the SATs, to help learn languages, and for music. If you didn't know, when people say "YouTube University," they are referring to the vast amount of free educational videos you can find on YouTube. Many of these YouTube videos are very simple for those just starting and can get more complex for more advanced students.
Mastering Services
While learning how things are done is always a good thing to have, If you're unsure, or if your release is time-sensitive, there are online services that can help! You can simply send your tracks over to them, (in the required formats, and breakouts), and their engineers can do it for you. CloudBounce is one of many you can use. If you're a member of CD Baby, you can get a deal! Click below for more information. https://diymusician.cdbaby.com/business/free-online-mastering/
Whatever you chose to do to create your music, make sure it's made ready by a master.