This guide is intended both for newcomers to the practice of drawing, and for those who are struggling to develop their ability to draw beyond the stereotypies and shortcuts they've reinforced while practicing. Many try to improve their drawing by learning to represent a specific subject - say, anime girls - but they do not truly develop the skill of drawing, per se, they only develop a more convincing and elaborate system of symbols and shorthand. Another common approach to beginning to draw "seriously" is to learn to draw forms, such as the routine offered by Drawabox and other programs, and imagine that the ability to represent subjects recognizeably will follow by rendering more complex forms, like a computer, without any other training, however this remains abstract until it can be applied artfully, which takes a long time to learn. It is difficult for this learner to verify the "correctness" of their forms according to their reference, so usually this ends up only half-learned: the vague bean forms popular in animation are ubiquitous because they create a convincing illusion of depth and movement with little capacity for error. It becomes a symbol in disguise. It's common for the hobbyist to complain that they can draw one thing just fine - portraits, animals, trees and flowers, interiors, comics in a certain style - but haven't learned to draw other things very well. The truth is, all drawing is the same essential skill of observation. Experienced artists have spent years developing the art of observation as well as of representation. Artists who draw well from imagination may have memorized many rules and facts about the human body, for example, but this is next to useless if they cannot accurately project this vision onto the paper - an act of sensitive imaging. Learning to observe will improve your ability to imagine pictures.
I will be dividing this page into three main sections: Section I is chiefly concerned with developing the shockingly easy skill of percieving reality the way it really appears, without the distortion of symbolism and shorthand. Section II is about things like technique, mediums, composition, painting, style, and expressive use of color. Section III is interested in what many people try to learn too early: matters like drafting and the study of anatomy. All sections will contain resources and advice and a crude roadmap which the reader can repurpose to their own needs.
It can be motivating to spoil yourself with sparkling new name-brand supplies, so do that if you wish, but to start with you only need paper and any writing utensil. I do recommend that you buy a sketchbook which you date the pages of as a record of your progress. Besides the means by which you draw, the next most important thing is a sustainable plan. I think you should fit this to your own needs because it's easier to stick to a plan you come up with yourself. I draw prettymuch first thing when possible, and I like to do an exercise on weekdays, three of an older exercise on Saturdays, and a full, planned composition on Sundays. I begin new sketchbooks with a Sunday Composition, as I call them, and then end them with a "better" version of the same drawing. Personally, I avoid looking at past pages until I'm done filling the sketchbook for a more "objective" reflection - I've rediscovered old sketchbooks from many years ago and wished I hadn't - some of that stuff I'd forgotten is shockingly bad compared to the stuff I do every day now. Your plan might look completely different from mine - as long as you draw often - preferably every day - and practice the right things, you will get better quickly.
Developing the skill of correct observation is a unique kind of practice, which I find more revealing and rewarding than other drawing skills. It is a unique dimension of perception that is more fundamental than our verbal thoughts. If I were to elaborate for too long, this section would be plagarized from Drawing on the Right Side of the Brain by Betty Edwards. I took a couple drawing classes in college from a teacher who apparently lifted her syllabus from that book without explaining any of the actual theory behind the exercises (even so, it was a rewarding class, even though I didn't do well). To summarize, the book supposes that the brain's left and right hemispheres, more or less, are tasked with handling verbal and spatial information, respectively, and that in our lives we process nearly everything with our verbal left hemispheres, which cripples our ability to draw - that is, to see - because everything is processed symbolically. The book offers exercises for training yourself to process visual information with your right hemisphere by giving the left hemisphere tasks that it will reject, and then offers important techniques for sighting. The book is an effective overview of drawing technique altogether, covering line, composition, landscape, portraiture, light and shadow, and color. This book is approachable to those who dislike dense and technical instruction, but is always purposeful and thought-provoking. The book's official site can be found here.
I recommend you consider buying that book - ideally a physical copy that you can annotate - but I will briefly summarize some of the book's most important exercises. Blind contour drawing is the process of fixing your eye on your subject, and slowly moving your hand as your eye along the edges and inner contours, without looking at the paper. This exercise is much older than the book, but as far as I can tell Betty's modified contour drawing exercise is original: it is a variation that introduces basic sighting techniques and the "permission" to periodically look down at the page. Another (again, very old) exercise is that of drawing the negative shapes between forms, taking great care to sense their proportions relative to each other.
As much as I recommend Drawing on the Right Side of the Brain, I recommend Kimon Nicolaides' manuscript The Natural Way to Draw. This book is far denser, and each section comes with a schedule of exercies, whose blocks can be adapted to fit over any period. I recommend using it after Drawing on the Right Side of the Brain. This book emphasizes impression as the vital foundation of "natural drawing", and its exercises concern expressing the gesture, weight, depth, and movement of a subject. It is also heavily focussed on depicting the figure, with indtructions for the study of different anatomical parts, and many exercising assuming access to a live model - though any live subject can suffice for most of them, or QuickPoses for its gesture exercises.
John Ruskin's 19th century The Elements of Drawing concerns itself, too, with this art of observation, and has seen many new issues since its initial publication, into our digital age. Instead of focussing chiefly on figures, most of its contents are concerned with the observation of trees and foliage, which is a favorite subject of mine, and the extended and detailed contemplation of trees has been of solemn personal significance to me since I was little. This book is composed of a list of exercises which are more primitive than those offered in either other book mentioned yet, so I would consider it less vital, however it can still be very useful. Like both other works, The Elements of Drawing is rich with fantastic quotations and thought-provoking notes about the nature of art and drawing. The language is obviously quite dated, albeit in a quaint way, which can make it slower to read. Here it is on Project Gutenberg.
Collage is hardly considered by those persuing drawing or painting, but it's hard to name a method that better trains an artist in composition, color, lighting, perspective, themes, and creativity. Buy a couple trash magazines and some sturdy thick paper or cardboard - maybe some black construction paper for framing - and begin cutting up formats and whatever intreguing things you find in the magazines. It's wise to keep some scratch paper on hand for sketching out compositions. Your vision will adapt every time you glue down another clipping.
The website www.handprint.com has impressively kept documentation on watercolor painting, from very detailed information about paints, pigments, paper, and other materials, to brush technique, to color, to sighting. It is an extremely dense document, recommended for those who enjoy highly technical information, so read carefully.