Ok, so you have the same camera as I do. For close up shots here is what I would suggest:
1. A macro lens would be best but even with your current 18-55mm lens you can get better results
2. For close up shots, you need a tripod and a remote shutter (or use the timer function on the camera). Pressing the shutter release jiggles the camera and reduces any chance for a sharp picture
3. Shoot as perpendicular to the glass a possible. Any angle will reduce sharpness and focus
4. Set your ISO to 100 for coral pics, and 400 or 800 for fish pics. If you put it in Auto, it will select higher iso settings and give you noise and reduced sharpness and clarity.
5. Shoot in Av (Aperture Priority). Set you f stop (aperture) to lower settings to get more sharpness and clarity. Play with the f stop to see how it affects your depth of field and sharpness. You change the f stop using the little wheel dealy on the camera. Once you are more comfortable with setting f stop, you can shoot in full manual and play with your exposure setting as well. But to begin, Av will get you good results.
6. Learn how to setup a custom white balance. You can read about it on the internet. Once you set a custom white balance, the color in your photos will come out much better and closer to real life
7. Use manual focus. To get the best focus possible, use the LCD screen, and zoom to 5X or 10X on there so you can really get close in to get the focus right.
Those few things ought to get you moving in the right direction. But don't expect miracles with the stock lens. A macro lens is a definite investment, but the results obtained will be far better than with the 18-55mm lens.