Real life » The best references are the real life scenes. For figure drawing, the cost of a professional model is usually high. But there are usually group life drawing sessions in some local museums, art studios or even bars one can go and draw for low cost. You can also try to draw your friends if they are willing to pose for you, or draw each other in a drawing group.
Alternative, one can draw people in the public like in a coffee shop, on a train or in a park. 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). Note many paintings in art books are reproductions by other artists, not necessarily a photocopy of the originals. And due to printing and many other factors, the colors could be quite different from the originals. If you have ever tried to take pictures of paintings in a museum, you will know that the colors viewed through the camera is very different from that viewed from your naked eyes.
Google Art & Culture is an exciting project that is collaborating with an increasing number of museums to provide digitalized high resolution artworks. Art Renewal Center is the largest online museum dedicated to realist art, including many pieces by John Singer Sargent, Anders Zorn and many others.
It is easy to blindly copy a master work, but if one don't actively analyze and think about how the master constructed the painting, then probably the rewards from master studies are quite limited.
Photo references » observing from photos has a lot of drawbacks, but this is also the most accessible approach. 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.
There are also fashion magazines and websites, but the quality for the purpose of drawing references could be varied, and some of them are heavily photoshoped.
The same holds for landscape photos. Especially if you want to paint with colors, be careful the colors in a photo might have been heavily modified (which is not necessarily a bad thing though). Even for photos that are not edited, some value information has already lost because the dynamic ranges of cameras are much smaller than human eyes.
The advantage of using photos as references is that you can use softwares to manipulate the references for better analysis. For example, one can turn the photo into grayscale or using curves to simplify the value structures.
Online courses and tutorials
Many artists will 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 very 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. I feel the interface for interaction between users is not very convenient and the quality of artworks has relatively higher variance.
- Instagram » a general purpose picture sharing app. It has a lot of limitations that are not good for sharing arts like low quality images only suitable for viewing from phones and restricted aspect ratios, but it has a large user base, and many artists post their drafts, painting process or finished works on it.