Let's imagine your canvas is in a rectangular shape;


Composition
Trace an imaginary vertical line a third of the way across the painting (on the left or right hand side). Here you position your main subject. Remember, it's this subject that must be highlighted. The painting must also be constructed on the basis of geometrical forms - this gives it structure.

Perspective
The horizon, seen from the point where your painting is in front of you, is a visible line if you're attempting a sea painting, otherwise an imaginary line.
Everything that is above : is as any horizontal line, coming together at their vanishing point on the horizon.
Everything below : likewise, the horizontal lines come together at the horizon.
Vertical lines : easy, they stay vertical (which is what cameras have such trouble with, owing to the curved structure of their lenses)
The vanishing point : The point where horizontal lines join. For example, the rails of a railroad converge at a certain point on the horizon line.

Changing values
In nature, looking at a landscape that to your eye has a foreground, a middleground and various more distant planes, like a range of mountains in the distance, you'll see the air becomes less clear with distance. If you're in the water, looking ahead of you, the phenomenom is even more apparent. A goldfish, for example, as your eye sees it, loses its color the further away it is from you, until it turns completely black.
Foregrounds : Can be portrayed with the greatest clarity,
Middlegrounds : Becoming more blurry,
More distant planes : gray-blues, colors are rendered more opaque due to dust or mist in the atmosphere.
Recommendation : This doesn't stop you, when painting a stormy mountain scene, for example, from depicting a ray of sunlight lighting up a village below. On the contrary, this is the element of the painting you should highlight.

Colors
The primary colors, blue, yellow and red, are the base colors, from which you create all the colors of your palette. You will also need black and white, to create gray.
Observations :
Black and white together give a slightly bluish gray. Black, yellow and white, will give a greenish-gray. Green and a touch of red produce a dull green. Secondary colors are obtained by mixing the primary colors according to the following diagram :

Depending on whether your mix is heavier in blue than yellow, you'll have a stronger or lighter green. The best way is to experiment with very small quantities and to judge the results for yourself.

For oil painting :
Turpentine helps to obtain umber colors (and also to clean your brushes). Linseed oil is good when you want to apply paint more thickly and to highlight luminosity.