Not complete yet!
I didn't learn to sing until I was in my twenties... I used to be told I couldn't sing, and assumed there was nothing I could do about it; until one day someone offered to teach me, instead of telling me to shut up... and I found that it is possible (and quite easy) to learn.
My problem (and I think it is the usual problem) is that I wasn't comparing the pitch I was producing with the pitch I was aiming for. So the first and most important stage is to find an instrument with a stable pitch, which you can play without using your mouth, and which can produce a sustained note. Pick a note within what you expect your range to be, and play it, and `la' or hum... change the note you are making up and down, and you should hear clearly when it becomes the same as (or very close to) the note you're playing on the instrument.
Now try another note... try a succession of notes, scanning your voice up and down to find it each time, and locking on to it.
The next stage is to find a simple song that you know and can play (or get someone to play for you). Play it slowly, taking time to match your voice to each note.