Yeah, that's definately a nice start.
Maybe the sig's size is too small, and that's why David can't be seen very well.
The lensflare is good, but too bright and it covers the figure too much.
Brushes are a good option as the other mate said. Or maybe try something different with the render (as you are a beginner, you can click on every effect and see how it will look, the better you become,the better you will know which effect fits with what).
Make some effects on the text.
You can try one of my
fav things - the Rainbow effect, which is few brushes, manipulated by vectorizing. The button is placed next to "Artistic brushes" (or,if they're not called like that, the button is on the board, u'll find it easy.Just look carefully)