This is a straightforward recommendation: if you have fun drawing, aren’t very good at it and would like to get better, Drawabox is an excellent resource. And it’s completely free.
Its distinctive compared to the other online resources I’ve used is a tight focus on teaching the principles of perspective, and on helping people to develop an intuitive sense of how something in the real world should look on the page. You’ll need to look elsewhere for things like sketching, mark making, colour, and light and shadow.
Just working through Lesson 1, and particularly the section on vanishing points, has changed my understanding of how pictures (and the world) look. It was one of those way-out-of-proportion-to-the-time-and-money-spent lessons that thrill and delight.
I’d recommend working through from the start, but here’s the page on perspectives and vanishing points, and below is the video to go with it.