Sometimes I like to try a different approach to painting. Here's a way to begin a piece with no preconceived idea of what the subject will be.
Start by wetting your paper and dropping some colour in.
Add more colour according to your whim. Tip the paper around to let the colours flow.
Tear up bits of tissue paper and place them randomly on the wet surface. You still have no idea where you're going and that's fine. Now let the paper dry. Go do something else. Have a coffee or go shopping.
Remove the bits of tissue and stare at the result. You now have the matrix for your painting. Organic forms usually present themselves to be developed this way. Make a thin mixture using all your original colours to begin isolating the forms you want to define. You will have a neutral shade to set off your shapes.
Keep going, adding by subtracting. This is known as painting negative space. You are painting what's NOT there in order to show what IS there.
Combine hard edges and soft edges. To keep an edge soft let it trail off with clear water.
Strengthen your composition with darker washes and glazes in the background. Add a few details but don't be too literal. When you can't think of anything more to do your piece is finished. Resist the temptation to overwork it. If you're not sure just put it away for a while and look at it with fresh eyes. That's always the true test.