To be honest, some people have a fantastic talent with drawing portraits or drawing from reality, but can't for their life draw from imagination, and some have the opposite issue and can't draw from life without having an urge to punch their pencil through various objects (and subjects). So, if you can't draw from imagination, perhaps that is not the best place to start.
Anyway, I can't really tell you how I started. I have always been drawing when I could, and I just kept doing that. Then I started picking up various techniques and ideas various places... I have tried both art classes, which never did me well, books on various ways of drawing, which can be somewhat boring at times, tutorials, speedpaint videos, glaring at people jealously because they had better skills than I had...
And I can't recommend any of it directly, because any of that might be helpful to you, but not to others.
But, well... Depending on your interests, I will recommend learning anatomy and proportions. You can get really far with no talent and no skill, as long as your humans have the right proportions (and thus will look vaguely like humans still), and from there, it will be a bit easier to correct your drawings than it would anything else. Shading comes later, colour theory comes even later.
So, get a good art instruction book (for once I will recommend Christopher Hart's 'Human anatomy made amazingly easy', it covers the basics without looking so good that you get discouraged), look it through, then sit down and try to remember what you just read. If you get in doubt, don't go for the book, instead, get up and look down yourself, it is far easier to understand how the proportions works when you are practising on yourself, and keep the rereading to other times.
Other than that, deviantart.com has a good amount of talent and many of those talents have made tutorials and guides to drawing pretty much anything, so that might be the best quality place for no money you can encounter outside of speedpaints and staring masterpieces down.