If you are leaning how to paint, learn how to paint clouds. Clouds are a classical part of landscapes and are also beautiful just on their own.
How to Paint A Blue Sky
Use a one inch landscape brush. Dip your brush in a blue/black color paint with a little linseed oil. Brush back and forth horizontally across the canvas until you fill almost all of the canvas. If you are going to have land with your clouds and sky, stop where you want the horizon to be.
Add some white and linseed oil to the brush and, starting where you left off, paint horizontally upwards towards the top of the canvas. Add more white and brush horizontally.
Keep on painting until you like what you see. The main concept behind this is that the colors shouldn't be uniform, but should be darker the farther away the sky is from the horizon.
How to Paint Clouds - Technique One
Start with a white color on a brush on the left side of your canvas. Apply the paint in a circular motion, moving to the right. Repeat as you like, making sure that the strokes are uneven.
To bring out a softer bottom on your clouds, wipe off your brush and circle your brush in the same manner. You can also pat the tops of the clouds, still with a "wiped" brush, for a puffier look.
Next, add more paint to your brush and circle-brush more clouds on top of your existing clouds. You can also pat the paint on as well.
How to Paint Clouds - Technique Two
Load your paintbrush with a white color on the top and a darker color on the bottom to create a shadow. Dab and swirl the paint the clouds so that the clouds have wispy ends. From time to time, paint the clouds with a purely white brush.
Regardless of the technique that you use, make sure that you don't repeat the same pattern when painting clouds. It can be easier to be repetitious than you might think.