Hi Fiona,
Here's a little story that some photographers might find a little bit sobering, if not scary!
Many years ago I asked my mentor, a very well regarded professional photographer, how long it takes to learn about light; he told me it takes about 10 years. I reminded him of the comment quite some time later and he told me that he changed his mind. He now reckons that it takes a lifetime! Some days I am inclined to believe him.
Light is the core element to making a subject look good and yet it is often the most misunderstood. That being said, the only way that you can get better at it is to practice, practice and practice some more.
With this photograph of the iris you get a distinct feeling that there was light on one side of the flower but not on the other. At a reasonable guess I would say that you are holding the camera in the vertical with your right hand on top and with an on-camera flash to the left of the lens. The darkness on the right of the picture is probably the shadow of the lens.
To get away from this problem you do need to take better control of the lighting. Rather than using the on camera flash, a better option might be to use available light such as daylight from near a window. To get the light even you can use diffusers (these are usually translucent fabrics or even something like tracing paper scattered light) and reflectors. White polystyrene foam panels create really good reflectors what if you want something a little brighter you can try using silver cooking foil.
Place the flower in some good light (this does not have to be direct sunlight) and before picking up the camera, explore what happens as you move the flower about in the light. Look at the areas that are lit it also those that are in shadow and see what effect the diffusers the reflectors have on them. Put these light modifiers to the left or the right of the subject but don't forget that you can also place the above or below.
Have a play and I think you will discover very quickly why lighting can be so complex. Don't be put off by it though… treat it as one of the best puzzles a photographer can explore.
I hope this is a help!
Cheers, Anthony.