Real life » The best references are the real life scenes. For figure drawing, there are usually group life drawing sessions in some local museums, art studios or bars. Alternative, drawing people in the public (coffee shops, subways, parks) is another option. If you are intimidated at drawing strangers, you can go to museums and use sculptures as references or consider joining some local group outdoor drawing sessions. For example, the Urban Sketchers seems to be a good community with many local groups and events near many major cities. For landscapes or still life, it is relatively easy to find real world references.
Master study » A good way to learn is to study the drawing or painting from established artists. The best way is to go to study in the museums. Alternatively, one can also buy art books (e.g. TASCHEN has many good ones), or use online resources such as Google Art & Culture, Art Renewal Center or even Art Station for pieces by the current generation artists. For masterstudies, try not to blindly copy, but also analyze and think about how the masters constructed the painting.
Photo references » photo suboptimal for painting references but very easily accessible thanks to the Internet. There are many websites such as line of action, sketchdaily and senshistock that provide random photos of human poses for drawing and other tools like timing. There are also more specific (and paid) websites, such as BodiesInMotion that uses high-speed camera to capture sequences of very dynamical poses (some with 3D models), and Anatomy 360 and Human anatomy for artist that focuses on anatomy.
I also use Pinterest to collect and discover photo references. Note many general photos online are heavily edited or captured with deliberate lighting setup, therefore not very good for painting references, especially for learning. For example, the light and shadow on faces might be too subtle to judge the 3D forms.
One advantage of photos references is that you can use softwares to manipulate and analyze them (e.g. turning them into grayscales).
Online courses and tutorials
Many artists share their painting processes or tutorials on YouTube. A lot of high quality courses are also available on various platforms or online academies. Many online art classes have two modes: one self-paced mode that basically provide access to course videos, and one
regular mode that follows weekly course schedules with homework assignments and feedbacks from the instructors. The latter is usually much more expensive (though still cheaper than going to art school) and with limited number of slots.
- Schoolism is a monthly subscription based platform. I watched two classes: Painting with Light and Color with Tonko House and Cody Gramstad and Essentials of Realism with Jonathan Hardesty, and they are both very good.
- New Masters Academy is another subscription based website that contains many multi-session classes and shorter lectures. Many of them look quite good, though I have not personally tried it.
- Computer Graphics Master Academy (CGMA) is a platform focusing on concept art and illustrations. You can only enroll the courses with assignments and feedbacks, therefore their courses are expensive.
There are many good books for learning about arts. I only list a few that I have read and would like to personally recommend.
- Picture This: How Pictures Work, by Molly Bang. A small booklet that illustrates the art of composition via only very simple and abstract shapes. Very interesting and insightful.
- Color and Light: a guide for the realistic painter, by James Gurney. A realistic painting does not necessarily be very detailed, and vice versa. This book illustrates some very important aspects for making realistic paintings.
- Morpho: Anatomy for Artists, by Michel Lauricella. There are many good art anatomy books. This is probably the one with lest text. There are not much instructions, but the illustrations are very beautiful, with indicators showing which muscle is which under different poses. I learned a lot by copying the illustrations and marking different muscle with colors.
- Framed Ink: Drawing and Composition for Visual Storytellers. A book about ink drawing, but also very good learning resources for compositions, value simplification, application of perspective, and more. The author has a few other books in the series which are all very good.
- ArtStation » a professional community focusing mainly on concept arts and CGs.
- Pixiv » the largest community for sharing Anime/Comics/Games related artworks.
- DeviantArt » a community for a mixture of all kinds of arts. The quality of artworks has higher variance.
- Instagram » also very popular among artists, even though not a dedicated art platform.