What many people mean when they say they'd like to learn to paint, is that they'd like to learn to paint realism—to create a painting that looks "real" or in which the subject looks as it does in real life. It's only when you're up close that you see the skillful manipulation of color, tone, and perspective used to create the illusion of reality.
Painting realism takes time. Expect to spend days and weeks, not merely a few hours on a painting. You can't paint detailed realism and also want to knock off a painting every afternoon unless you're painting a small canvas with something simple like a single apple.
• How to Create Time for Painting
• How Long Should It Take to Finish a Painting?
If the perspective is wrong, the painting won't look right, no matter how beautifully it is. Get the perspective accurate before getting into the fine detail. Check the perspective regularly as you're painting to ensure it remains accurate.
Shadows aren't solid black. Shadows aren't shapes of darker color painted right at the end after you've done everything else. Shadows aren't the identical color or tone in all areas of the composition. Shadows are integral parts of the composition and should be painted at the same time as everything else. Spend as much time observing the subtle shifts in color in shadow areas as you do in the non-shadow parts.
• How to Paint Shadows
Don't take a single photo and turn it into a painting. Not because it's "cheating" but because your eye doesn't see the same as a camera. Your eye sees more detailed color, your eye doesn't frame the scene in standard proportions, and your eye doesn't have a depth of field that's dependent on a setting. A realistic landscape will be "in focus" all the way to the horizon, not blur out of focus as a photo with a narrow depth of field will.
Color isn't a set thing—how it appears is relative to what's next to it, what kind of light is shining on it, and whether the surface if reflective or matte. Depending on the light and time of day "green" grass can be quite yellow or blue; it's never a simple match to a single tube of green paint.
A subject painted with great technical skill isn't enough to make a good painting. The choice of subject needs to speak to the viewer, to grab their attention and compel them to keep looking. Spend time considering the composition of your painting, what you're going to include and how you're going to arrange it. Work it out before you start painting and you'll save yourself anguish in the long run.
Painting realism isn't about copying the world as it is. It's about selecting and composing a slice of reality. Canaletto's paintings of Venice, for instance, may look real but in fact, various buildings are painted from different points of view to make a stronger composition.