Copied and pasted this from another thread I answered, too lazy to re-write it lol. Still, I hope it helps!
If your camera has auto-focus, which it probably does, a good way to tell if you're going to take a good picture before you actually take it is to push the shot button down half-way. This will make the camera focus before it actually takes the shot. You can push the button down half-way multiple times and it should get a little difference focus each time.
Here is a picture that is incorrectly focused. It is clearly focused on the rocks, rather than my fish. It is easier to get the camera to notice the fish if there are less things in the background.
Here's a picture that is clearly focused on the fish. Note this minimalistic background. It is of course possible to get pictures with a busy background, but if you are trying to get a feel for taking well-focused pictures it will be easiest if your fish has got the most contrast.
Bright lighting will also help with your pictures, as more light = a higher exposure which means a faster shutter speed. A faster shutter speed will result in pictures that are less blurred.
Here is an example of too slow a shutter speed. The fish appears blurred, rather than crisp. Don't worry - you will normally get a ton of these shots if your fish is zooming around!
Here's an example of a picture with a correct shutter speed. Chalupa was zooming around, but the picture was taken fast enough that it froze him in time. It's all about luck and getting the right light.
I hope this little crash course in photography basics helped a little bit! The most important thing that I've found when it comes to taking a good picture is the 'push halfway' technique. Good luck, and if you've got any more questions feel free to ask!