The exposure is really good, both the grass and sky are correctly exposed. The composition is solid, maybe cropping the picture would be better but it’s good either way. The camera settings you used are decent too, f/8 to capture the whole landscape in focus and 1/500s shutter speed to mitigate camera shake. And lastly, the whole idea of the image is beautiful, bright blue sky and a green field with a little rainbow in the middle.
Ways to Improve the Image
Looking at this image it’s obvious that something is missing to give it that wow factor - That missing thing is contrast. There isn’t enough separation between the ground and the sky, especially between the sky and the clouds. Adding a bit of contrast would make the whole image more dramatic and give it more depth. The colours of the rainbow would also pop off the screen more.
In the image below I have edited the original by adding contrast and also some HDR improvements plus a vibrancy increase.
Cropping is a very important factor in photography. If you don’t frame the photo good while you’re taking it, you can crop it to achieve a better look. With this image, a vertical crop would look awesome, you’d have the grass, sky and rainbow in one image without having too much empty space like you do in the original. It would also compliment the rainbow since it would look bigger and more significant. See what I mean below...
The colours in the original photo look a little bit muddy I.e the greens are more yellow than green. In your image editing software you can try and do some correction, mainly taking away some yellows/greens in the tint and adding some cyans/reds.
As you can see in the examples below, colour-correction is a subtle change but it makes a huge difference.
There are lots of things that you can quickly and easily learn with practice when you are starting out. Check Excio membership offers and start improving your skills with personalised constructive feedback reports on your photos.