The reason you want to do a 'fishless cycle' is because you can kill your fish if you just add them to a tank that doesn't have a proper balance of naturally occurring chemicals. What you are doing is basically adding the stuff that a fish would add from its poop, without the fish!
Do you have/can you get test kits for ammonia/nitrate/nitrite? If you have these then you just need to buy ammonia - you can get this at a grocery store. Then it is just a matter of putting in some ammonia and testing the water regularly and that link is pretty easy to follow in terms of what numbers you are looking to achieve (or maybe you mom or dad could help you decipher it).
You are going to want these test kits at some point anyway, so do try tis way.
When I did my fishless cycle, I didn't know how to DIY with ammonia, so I did use a product called Cycle that will add stuff to your tank for you. A lot of people think Cycle is a waste of money, but it did work for me. I also put in a bunch of java moss that had lots of 'gunk' on it from a healthy tank at my LFS to get some bacteria going. In fact, I set up my tank with all its plants, and let it sit for about a month before having the water tested and adding fish.
Another option would be getting some used gravel and water from a disease-free tank - maybe your local fish store would help?
Now I'm certainly not very experienced, so if I'm steering this kid wrong, someone please say so!
