One of the most difficult skills for a beginner to master is how to simplify your painting. 

Painting everything in a scene will make it look too cluttered.

Identify what is important and simplify the rest.

If you are painting plein air or from a photo reference, survey your scene and think first about what you are going to cut - then cut some more. Doing a few simple sketches may help you decide what should stay and what needs to go.

If you are painting from a photo remember the photo will have everything in focus, which our eyes will not. For a more realistic painting the focal point should be in focus, the background slightly out of focus.  A few brush strokes painted out of focus can represent foliage in the background for example, which will look realistic and save you hours of work.

Simplify your palette

Stick to a few colours and mix these to produce different values and tones. Too many colours will make for a confusing painting. Different values of the same colour will make a more elegant painting and will add more to the painting than adding more colours.

A mixture of light mid and dark tones of green is more effective than adding a number of different colours.

Simplify your technique as well. Think about every brush stroke and make each stroke count. Mindlessly painting until you get it right will result in a muddy mess and will take forever to produce anything like a decent painting.

The painting below uses a simple palette and the trees in the background are slightly out of focus