I have been reading quite a bit of forums from different subjects. I am still very unsure by what a water change is. I've seen 25% or all different kinds of numbers, but still cant figure it out. I feel i should be doing this somehow, but i am very unsure of how to do this.
Thanks in advance.

Well, a water change is exactly what it says, changing some of the water in your fish tank.

Keeping fish in a tank is a huge compromise to what it would be like in their natural environment. In the rivers, seas toxins like ammonia, nitrite and nitrate don't affect fish because of the amount of water there is. (ammonia being produced from fish waste, nitrite a byproduct of ammonia, both are very toxic) If your filtration is working properly, you shouldn't have ammonia or nitrite present . However, after the bacteria in your filtration has dealt with ammonia and nitrite, you then have something called nitrate which is left over. This isn't as toxic but it still has to be kept under control. The easiest way to do this is to do a water change. The smaller the aquarium, the quicker the water will become polluted. It is recommended that you do one water change a week. If you are keeping your fish in the correct sized aquarium, a 20% water change is adequate. If you are overstocked, or have a small aquarium, you might have to carry out a larger one.

So to reiterate, once a week, siphon some water out of your tank and replace it with fresh water, remembering to use a water condition to remove additives such as chlorine that is added to tap water. It really is as simple as that

Well, i guess that does make sense. When the water gets low, i add new water and use the chlorine cleaner. But i have never taken water out and added new water in. Opal has just been transferred to a new tank and so every time I open the tank lid, she quickly hides. I hate that she is so scared of me. I dont make any real sudden movements around her. any advice?

If the lights are on, tan switch them off before you open the lid. Unfortunately, Oscars can be a little scared at first. Hopefully he will get used to you in time

Penn is quite right about water changes to dilute the amount of nitrates that are a by product of the nitrogen cycle. The problem with nitrates is that the rising levels can creep up on you slowly if they are not checked regularly. Nitrates are much harder to remove than ammonia and nitrite as the process (called denitrification) is much more complex than nitrite and ammonia removal. Briefly it involves the water coming into the contact with the media for much longer in an oxygen deficient environment. This is extremely difficult to do and involves a much larger filter running at very slow rates i.e. drips. The far easier way to deal with nitrate removal is to dilute by replacing the water with fresh. You should though always check your tap water for nitrates before you add it to the tank as tap water is not always nitrate free. You could choose to use some sort of nitrate removal in your filter but this normally involves recharging the chemical with salt. Penn is also right to say that nitrate is not toxic however if it is left to build up to high levels over a period of time it will cause your fish stress and their health will suffer for it. In my opinion it is a major cause of the stress that brings on HITH. 20 % is absolutely the correct amount to change each week.Although this maybe a chore think of it as the time you might spend taking the dog for a walk or looking for your cat when its disappeared. By the way achieving less than 20ppm nitrates is fantastic for hungry Oscars as long as you strive to keep it under 50 % your fish should be a fine.If you wish to find out more about denitrification and the nitrogen cycle just type them into google and youll find out all about it. Watch out though it can get a bit heavy and may make you think youd listended to your biology and chemistry teacher at school