Do you have a filter and heater? What size tank is it, and what fish are you planning on getting?
I strongly urge you to read up on the aquarium cycle and do a fishless cycle on your tank before adding any fish.
Here's an article on cycling to get you started:
For some reason, many people at fish stores will tell you to let the tank run for 48 hours before adding fish. This really accomplishes nothing. To cycle a tank, you need an ammonia source and a good liquid test kit as well as plenty of patience. It can take two to eight weeks to cycle an aquarium.