Well, my art professor once said a thing I never forget ^^
"Paint what you want, literally. If you suddenly think, that fish should be yellow, make it yellow. If you thin, you should paint from here with the hand instead of the brush, do it. Your instinct is better then any planning".
And I have to agree, the pictures get way more beautiful, inspired and original

The second thing is:
If you can, then build the canvas yourself. You can make a perfect rectangle or a trapeze or make it a giant picture or really small. But you can definitly make the picture you want to make and won't get limited by the space and form of a bought canvas.
The third thing is:
Colors and Brushes.
Mix colors. And I don't mean a color, but the stuff the color is made from. I once made a painting with black ink, Acryl, watercolor and blood. Believe me, it looks totally awesome

Also have a wide variety of brushes, maybe even a palette-knife. And show the bravery to try and use them all, you will realize that they all have their good points.
The fourth thing is:
As nice as it is to have a absolute worked out plan for a picture, don't do that.
Take inspiration, have a rough idea (like making a still life with mushrooms) and then just start and look what comes out.
I mean, when I thought about making a still life with a kiwi, it became a balcony scene in Paris and a friend of mine ended from a orange still life on a orange plantage landscape. Another girl even made a totally erotic looking picture of a cut open fig.
If you keep open a back door then awesome things can slip in.
And the last step:
Have fun while painting, then will everything be ok

Enjoy the chance of painting and knowing that someone will happily take it (don't doubt the last one even a second. Someone will take it, and if it is you yourself

)