Don't panic. This is actually good and perfectly normal. The tank is new and it's going through it's nitrogen cycle. That means all the good bacteria that you need to convert the ammonia and nitrites into nitrates so your fish stay healthy is establishing itself. This is going to take a few weeks, and I recommend you hold off on getting any fish for a while.

Here is a really good article on Fishless Cycling to help you understand what's going on and what to do and what to watch for:

Fishless Cycling - Article at The Age of Aquariums - Tropical Fish