Streaming platforms don't accept uploads directly from artists. Between your master and Spotify there is always a digital distributor: a company that delivers the file and its metadata to each platform, handles the release identifiers and collects the plays to pay them out to you later.
The usual model today is an annual subscription with unlimited uploads, and the key difference from a traditional label is that you keep ownership of your music: the distributor delivers and collects, but acquires no rights over the work. Even so, the specific terms vary between services and are worth reading before you sign.
What to have ready beforehand: the master as a full-quality WAV, never an MP3 — the distributor encodes from whatever you hand over, and starting from an already-compressed file degrades the result twice; the square cover art at high resolution, with no text that becomes unreadable at phone size; full composition and performance credits, which are what determine who gets paid what; and, if there are third-party samples, clearance settled before delivery.
On timing: deliver three or four weeks ahead. Distribution doesn't need that — it takes days. The platforms' editorial teams do: they need that margin to consider the song for their playlists at all. Releasing with two days' notice closes that door before you've tried it.
And before any of that, make sure the master is up to it: the mastering guide covers what to do and what not to, and LUFS and loudness covers what level to deliver at for streaming. Releasing is the easy part; having the song survive the comparison is the other one.
Disclosure: the DistroKid link on this page is an affiliate link. If you sign up through it, Fix The Mix receives a commission and you get the stated discount. It doesn't change the price versus the public offer, and it doesn't shape anything explained above, which applies equally to whichever distributor you choose.