Do you have test kits for Ammonia, Nitrate and Nitrite? If so, what are your readings?

Do you have any live rock? If so, where did you get it - from an established tank, mail order, ?

Your tank needs to be cycled. If you're using fully cured liverock from an established tank and it didn't spend much time being moved from one tank to the other it may not require much time at all. If you're starting with uncured liverock then there may be a lot of die off and the ammonia levels could end up killing anything you put in there right now and it could take a few weeks. Something to keep in mind is that patience really does pay off in this hobby - only bad things happen fast. Take this time to read up on the fish or corals you would like to keep and make sure that your tank meets their requirements and that they're compatible with each other.

When your test levels show that your tank is cycled add things slowly, the tank will need to adapt to the bio load of adding a fish. Test again after adding your fish then once things balance out then add your next fish or coral. An 8 gallon system is pretty small and can be quite fragile, definitely be mindful of that as you make changes to the tank.