I found http://drawabox.com/ super helpful when you're just starting off. You begin with the very basics: lines, ellipses, boxes and then iterate from there drawing boxes and cylinders in space. Followed by plants, animals, everyday object etc. You can also get feedback in the associated subreddit.

On youtube Sycras channel was the most useful for me. He seems to genuinely care about providing helpful content for all levels, not just selling a product. I especially like his multi part series on specific topics e.G. "Foundations of Light and Shadow".

He also runs a quite active forum.

https://www.youtube.com/user/Sycra/ http://forum.sycra.net/forum/