I know about the nitrogen cycle and stuffs, but i have no idea of maintaining these chemcial substances in the tank.
What should these ammonia, nitrite, nitrate values be?
How can i acheive them?

a healthy biological filter will take care of the ammonia and nitrites. You will then be left with nitrate. This is much less toxic and can be controlled by carrying out regular water changes.

Ammonia and nitrite should always be Zero, you don't want any present in your aquarium, if it is present, it can be very dangerous for your fish.

Looking after your filtration this to is absolutely paramount. Never wash the media out in tap water, always in existing tank water. if you look after your biological media, you shouldn't have any problems with toxins

thank you very much,
and how about nitrate, i have heard of "ppm", what is that?
do u have a webpage saying about that?

PPM stands for "parts per million" as in water.

nitrate is a byproduct of ammonia and nitrite. So fish go to the toilet and produce ammonia, the bacteria in your filtration come see me ammonia and then nitrite is produced, then some more bacteria consume the nitrite and you have nitrate left over. There isn't any bacteria in your filtration to actually remove nitrates, this is why we carry out regular water changes.

but can u tell me how much ppm of nitrate should i have in my tank?
lower the better?
what is the maximum?

Ideally, you wouldn't have any nitrate, however, because we have limited amounts of water, nitrates are always going to be present.

As for how much nitrates, you answered your own question really, the lower the better. For long-term health issues, you really should keep your nitrate levels below 40 PPM. Obviously the level of nitrate in your aquarium will greatly depends on how you stock your aquarium and other reasons such as how much you feed your Oscar.