Cycling is the general term for establishing a bacteria colony in your filter. The colony will contain two types of bacteria, one that processes the ammonia from fish waste (very toxic) into nitrite (less toxic), and one that processes that into nitrate (relatively harmless except in high concentrations).
There's a few ways to do it, all of them will call for a test kit of some type, preferably a liquid drop kit (more accurate, easier to read, and despite being more expensive to buy, they last much longer than strips and save you a lot of money in the long run).
If you've already got fish (usually caused by trusting advice from pet stores), you need to do frequent and large water changes. Any ammonia or nitrite is bad, any of either above 0.25 parts per million is immediate concern, and anything above 0.5 is an immediate danger and calls for drastic action. Keeping it low will require daily water changes, possibly two or three a day, as much and as often as necessary to ensure that ammonia and nitrite remain below 0.25 at all times. This is called a "fish-in cycle", and may take a month to two months depending on your stock levels. If the filter isn't strong enough or the tank big enough for the fish (other potential results of pet store advice), it may never finish until some fish are removed.
Preferred is a "fishless cycle", which involves using an artificial ammonia source for the same effect fish waste would have in a fish-in cycle. The ammonia can come from household ammonia, rotting fish food, cocktail prawns, or even pee if necessary. No water changes necessary except in certain circumstances, since there's no fish to kill, nitrite and nitrate are allowed to spike out of control. It typically takes 3-4 weeks. The process is a bit involved, and is described in detail in the Fishless Cycling sticky:
[URL="http
/www.fishforums.net/content/New-to-t...shless-Cycling/"]http/www.fishforums.net/content/New-to-t...shless-Cycling/[/URL]
More general information about what cycling is and how it works, and what to do when you already have fish:
[URL="http/www.fishforums.net/content/New-to-t...eady-have-fish/"]http/www.fishforums.net/content/New-to-t...eady-have-fish/[/URL]
Edit: Some people cheat and just link to the existing information
Thanks guys! I'll read everything!
