How long has the tank been set up?
Have you completed the nitrogen cycle yet?
What kind of fish?
If this is a brand new tank -- take the fish back to the store where you got it. It'll just die in the tank.
If the fish store sold you a damsel to cycle the tank, take it back. You can cycle the tank with any piece of shrimp. Go to the grocery store and head for the fresh deli department. Buy a single fresh (uncooked) deli shrimp and toss it in the tank. After about a week, you can remove the rotten carcass and flush it down the toilet. There will be enough rotten meat in the tank to start and finish your initial tank cycle.
Wait about 4 or 6 weeks for the tank to fully cycle. Test daily for ammonia and nitrites. Both will go up to very high levels and then slowly drop back down to zero. When both have dropped to zero, you can test for nitrates. Determine your nitrate levels. Usually a 50% water change is in order at this point. Try and get nitrates under 10ppm. 0ppm or 5ppm would be better.
Only AFTER that initial cycle is complete -- THEN you can introduce a new fish.
Putting a new fish in a new and un-cycled tank is cruel and not necessary. You can cycle the tank with a deli shrimp.
If your fish store sold you a fish to use as a sacrificial fish to cycle the tank, I would NEVER return to that store. They are just selling you fish to get your money and don't give a crap that it will probably die.