First: invest in a liquid test kit
Second: test your water per kit instructions
Third: Every time you get an ammonia reading over 0-.25 (some kits will show .25 ppm of ammonia when the tank is actually closer to zero), you will need to do a water change. Say you have 1 ppm of ammonia on your first test reading. If you do a fifty percent water change it should be about .50 ppm. If you do a ninety percent change, it should be lower.
You will test your water daily. For a while you will only have ammonia readings. Eventually your ammonia should go to zero, but you will have nitrIte readings. Then, several weeks later, your nitrIte will be converted into nitrAte which is much less toxic.
If at all possible, return the fish so you can begin the much faster, much more humane fish-less cycle. There is more information in this section of the forums on both methods of cycling.