Put it in Manual mode and find these settings and fool around with them independently and see the results you get.
Here is a quick very general run down of what they do:
Shutter speed: 1/X, higher the number X the faster the shutter. Fast shutter will freeze moving objects like fish but you will also get less light to the sensor. Lower number = more light but if you get too slow you will start to see camera shake unless you have a tri/monopod.
ISO: higher the number the more the camera is boosting the exposure. You can use this to compensate exposure for other settings you are using (i.e. boosting ISO to make up for lost light with fast shutter speed to freeze fish motion) on most cameras once you get beyond 300-400 ISO there will be the addition of digital noise (kinda looks like a snow storm of random white pixels particularly in shadow areas) the higher the number the more noise, however there are programs and settings in camera that will reduce this noise but will make your picture overall a little less sharp as well.
Aperture (F-stop): This one is a little more confusing. This controls the size of the hole in your lens that the light passes through, so logically the bigger the hole the more light the sensor gets. However the numbers are a little counter intuitive. The smaller the number the bigger the hole, bigger number smaller hole. The aperture also controls the depth of field of the picture (how much or little is in focus) the bigger apertures (smaller F-stop #'s) give you a small depth of field (less stuff in focus) and the smaller apertures (bigger F-stop #'s) will give you a large depth of field (more stuff in focus). This one you really have to experiment with to get a feel for how it works. Frame a shot with an object in the foreground middle and back ground and take it with the biggest and smallest aperture your lens has while keeping the framing and the exposure the same. It will become really apparent what it does.
White Balance: Adjusts the colors the camera "sees" to compensate for color casts cause by various light situations. Cameras vary on how they let you adjust this. Most of the time there will be auto(works most of the time),various presets (cloudy, sunny, tungsten), and a range of Kelvin values that work just like the ones we use for our lights; higher numbers are more blue while lower numbers are more yellow.
Well those are the basics. There is a photographer that made a free online class that is pretty in depth and goes through everything a beginner photographer would want to know that he posted on his website here:
http://www.alexandrebuisse.org/resources/photo-class
Have Fun!