Adrian,
So, this is your first toe in the water? Already, you have glimpsed the difficulty of this sort of photograpy. Since these shots are from your garden there is no reason that you can't repeat these shots to improve and experiment. I'd love to say these are a great first attempt, but let's say they are the foundation for improvement. In the first one, we can talk about technique and composition. I think the red flower and green leaves can be considered complimentary colors and that helps make the flower stand out; that's often good. In this case, the red seems flat and washed out, as if it is overexposed. Do you shoot in raw mode? If not, you should. Capturing good detail is in red flowers is a bit tricky and it is easy to overexpose the red channel. Do you use the in-camera histogram to assess the exposure? You should. If you don't know about histograms look on the web for a tutorial.; the histogram is the greatest advantage of digital photography over film. You can pretty well nail your exposure, or at least know that nailing it is impossible because of difficut conditions. What ISO did you use? Try setting it at 400 or 800 for a start, particularly with a 70-300 zoom. Try to get a shutter speed of 1/500 or better. It's hard to judge how well this image is focused. Focus, shutter speed and aperture are difficult to control in the graden when even a slight breeze can throw you off. However, in your own garden you can go out early in the morning when it is calm and the light is less contrasty. Or, late afternoon light is often easier to work with.
Discussion of composition could fill a book. But in this case we could start with selection. There are distracing elements just above and to the right of the main subject. Also, the red one in the background, lower left. Various techniques can be used. For example, you could temporaily move the offending blossoms. Or, in some cases, using a low f-number can help to blur out the background. And usually it's a good idea to avoid cutting off part of the flower. Composing a good flower picture is iIMHO very difficult. If you completely isolate the blossom, the picture can appear stark and clinical. Or as in the case of the second image, including several blossoms can make it difficult for the viewer to crasp which is the subject or jst where to look.
Just a few suggestions for your your consideration.
-Nat