Well, you'll need to monitor your water in order to know when it's ready for fish. If you haven't yet read about the cycle, please do. Information about it can be found in the Beginner's Articles at the top of the Freshwater Beginners board. If you don't have tests for ammonia, nitrite, and
nitrate (as well as
pH), you should get them. There are test strips and liquid tests. If you want accurate and reliable results, get the liquid tests. Aquarium Pharmaceuticals makes great liquid tests for ammonia, nitrite, nitrate, and pH, and you can buy their tests online or at your local fish store.
The cycle may take on avergage 4 - 6 weeks, but that's just an average, meaning it may take longer or shorter than that. You don't measure the cycle by weeks, but by water parameters such as ammonia, nitrite, and nitrate. When a tank is cycled, its ammonia and nitrite both equal 0. Nitrate is at some number, though it can be removed from the tank with a larger
water change at the end of the cycle. Once the tank is cycled, you shouldn't have any ammonia and nitrite. The only compound you'll have from that point on will be nitrate. Nitrate can generally accumulate up to 40
PPM and be "safe", but I'd personally not let nitrate accumulate above 20 ppm. You remove nitrate from water by regular weekly water changes of 15 - 30%, depending on your fish load.
But that's all for now. Read up on the cycle first

Then ask if you have any more questions.
P.S. One last thing: As I have said above, even when your tank is cycled, you can still get an ammonia or nitrite spike IF you add too many fish at once. Which is why it's important to stock a tank GRADUALLY, adding only few fish at a time.