Once the bio-filter is established with the right bugs, it remains self replenishing...
Unless your water temp and/or pH values get radically out of range..... or you let it dry out for several days...
You can buy pre-prepared "beneficial bacteria" .... but there's a real question mark as to whether or not they actually work.... in any time that it would have taken for them to become active naturally...
That's the thing Davina.... the bacteria are naturally occurring... you just need to create the right environment for them and wait... they'll do the rest in 4-6 weeks...
Using filter gunk/water from someones aqaurium (that already contains the bacteria)... or seeding with system water and/or gravel from some helpful AP'ers system... will help shorten the time to "cycle"....
Or you can use "fishless" cycling methods like Seasol, Urea balls, urine (your own... if you're not squeamish about it).... some use dead prawns/fish....
Or you can just add fish... and juggle the balls and roll the dice.... just kidding....
If you cycle with fish... you need, at the very least.... a test kit, spare water, for water changes, and the time to test DAILY.... and to report your findings here for guidance....
Oh... and the last point is important... be prepared to seek guidance... it's free and readily available around here...
Most of the fish "kills" are those that usually go it alone... and don't post until it's too late....
By the way... welcome .... read through the forum.... knowledge is power....
the topics is Sludge for New Systems, in general system discussions 
sorry
) . Is this going to be help start things off?
I had two long relieves in 1000 litres and two days later the ammonia and nitrite went off the scale.