Don't worry too much. Think of all the fish you will give happy homes to after you figure this out. Take the last cory back, read the "cycling" post, and read the "freshwater fish profiles" to be sure they fish you want to keep all like similar water conditions.
Buy a ph test kit and find out what your water is like. That will help you decide what fish will do well in your tap water. Do NOT be suckered in if someone at the fish store tries to sell you something to adjust your ph. The swings back and forth that that causes are more stressful to the fish than just adapting to a slightly high or low ph.
Buy an ammonia, nitrite, nitrate test kit and use it to determine when your tank is done "cycling".
When you're cycled, start adding fish, but instead of just getting the fish used to water temperature, by floating him in the tank in his little bag, I like to:
Get a clean glass or pitcher, and my fish net. Open the bag with the fish in it. Hold it so that it floats in the tank, but the water and fish stay in the bag. Use the glass to pour a tiny drizzle of tank water into the open bag. Wait a few minutes. Add another tiny drizzle. Wait a few minutes... When your bag is getting so full of tank water that it is hard to hold, net the fish out of the bag and put him in the tank. Pour the bag full of fish store water and tank water down your sink. This way you get the fish used to your water chemistry as well as water temperature, and you don't pour nasty, germy, fish store water into your clean tank.
