I just cycled my 30 gallon freshwater tank in two weeks! You're tank needs to go through a cycle to build up beneficial bacteria that will eat up the ammonia your fish produce through waste. With out beneficial bacteria, you will have an ammonia spike and it will kill all of your fish because ammonia is poisonous.
People start off adding ammonia in their tank to start this process. You can use PURE ammonia, without dyes, surfactants, perfumes, or soap. You probably can find this at ace hardware stores. Since this is hard to fine, you could also use cocktail shrimp (made for human consumption) for ammonia. As the shrimp rots in your tank, it will release ammonia. I had three in my tank, in a nylon stocking.
Be sure to invest in a good testing kit (the liquid kind, the test strips dont usually give good readings.) Test every other day.
When you set up your tank, just the way you like it, with plants, gravel, filter, and a heater you can add the shrimp or ammonia. The first week your ammonia should spike, then fall. Then your nitrites should spike after that, and last for about a week. Once both ammonia and nitrites are down to zero, your tank has cycled. However, your nitrates are going to be high (which is toxic to fish too), so do a large water change, about 75%. Then you're ready to go. Cycling can take 3 weeks to a month to complete, however, it goes MUCH faster if you have a lot of live plants during the cycle!
When using shrimp, there's not measuring or having to remember to add drops. You just leave it be and let your tank cycle itself.
Good luck! Remember patience pays off!