The very first thing you want to do is make sure your tracks are exactly how you want, and then export the tracks as a WAV file as that will be necessary to publish.
Next, you want to register with Songtrust. Songtrust is a service that allows you to register yourself as a songwriter and the owner of songs, and then they will collect royalties on your behalf.
Songs Trust will give you a guide on how to go through their sign-up, which includes registering for a PRO (Performing Rights Organization).
PROs such as BMI or ASCAP. They allow you to register your songs and claim ownership of the composition of a song. a composition of a song could include lyrics on a napkin, but most likely will include chord structure, melody, rhythm, etc. Your PRO will collect songwriting royalties and public performance royalties and give them to you. Getting approved by one of the organizations can take up to a month.
While you are going through that process you should be thinking about a distributor. They are the ones that can actually get you on the streaming platforms. The best distributors I've seen are CD Baby and Disrtokid. CD Baby is significantly cheaper than Distrokid however CD Baby takes a cut of your royalties. Distrokid does not take royalties but it can end up costing a lot more especially for small artists.
Signing up for either of these distributors is free and easy. When you get to the stage of publishing your music they give you a walkthrough and make sure you have everything you need to get your music approved. You will need to have your music in WAV file format to submit it. You also have to have cover art for submission. They will both ask you to affirm your ownership of the music. You do not technically need to be registered with any of the previous organizations mentioned to get your music approved and distributed, but I would recommend you do so to be secure.
Another step you could take which is not required, is registering each of your songs with the US Copyright Office. In case you ever got in a dispute where someone was profiting from your work and your work was not being properly compensated, to easily sue them, you would need to be registered with the US Copyright Office.