There are a couple of ways of cycling a fish tank Emma.
But first of all, "cycling" just means to introduce a bacteria scource which will colonise your filter media and then break down your fish waste (ammonia) into less harmful substances (nitrAte)
This is called the nitrogen cycle....ammonia (lethal to fish) is released by the fish, broken down into nitrite (harmful to fish) which is then broken down nitrate (only harmful in large amounts) which is removed by water changes.
The two ways of doing this is
1) the fishless cycle......where a scource of ammonia is added over a period of days...usually from a shop bought concoction (although i've used fishfingers and even urine can be used !!!). The water is tested daily for ammonia and nitrite which will eventually go down to zero. It is then safe to add a couple of fish. Continue monitoring the water for a week or so and then add a couple more if all is well....etc etc
2) The non fishless cycle.....this is where a small amount of water and gravel is added from an existing mature tank to the new aquarium. The gravel and water will contain enough bacteria so you can add a few hardy fish almost immediatly. Some people will often rinse out a filter sponge from a mature tank into the new aquarium.....this will give loads of bacteria.
Whatever you decide (i always go for no 2 but that's just my personal choice), don't add guppies. These are now being bred so much that the gentics of them are very week and therefore the fish just ain't hardy anymore.
Come and have a look at this forum....very friendly and helpful
http://www.totalfishkeeping.com/index.php?
Good luck