Assuming it is a filtered tank it will need to be cycled. You need to read up on fishkeeping and get to know what you are doing before you put fish in there, otherwise they are almost certain to die.

Until you know that you can resist the urge to pee in it.

Usually after you fill the tank, you should wait overnight. At least that's what I did when I had fish.

If you care about the fish living, a week is a good time period. Very irresponsible to do it the next day! Bacterial colonies need a chance to start to grow. If you add too many fish at once, they may start dying. Buy a book, and read before you play with their lives.

What I do is wait for about 3 to 4 days, then add one fish. Wait for another two weeks, and add another. At the end of 4 weeks, the tank is usually "cycled", meaning it has enough good bacteria in it to handle more fish. At the end of 4 weeks, you can 2 more fish (depending on how big your tank is). Don't overstock your tank. A bigger tank can hold more fish, but a bigger tank also needs more time to "cycle" completely. One month (4 weeks) is usually the safest length of time to wait before adding the rest of the fish you want. But again, add only 1 or 2 at a time. Tips---1) the warmer the water, the faster your tank cycles, 2) do not overfeed your fish, 3) if your tank gets "cloudy" during the first week or so, that's normal because bacteria populations are adjusting, 4) if your tank is cloudy, don't change the water---leave it alone, it will clear up soon. Feed lightly, and 5) always check the ammonia level in your tank----during the cycling period, the ammonia in your new tank will rise. Ammonia is poisonous---if the level is high, do a partial water change immediately. That is why you only want to put one fish at a time over a period of one month. You can control the ammonia level better. Use an ammonia test kit (from pet store) to check ammonia levels. After one month or so, when the "cycle period" is over, ammonia level will drop to almost zero on your test kit----the good bacteria will have built up big enough to "consume" the ammonia in your tank. Once your tank is cycled, do a 10 to 15 percent water change once a week to keep toxins down. Remember to use water that is the same temperature and is dechlorinated (you can use a chlorine neutralizer). Good luck and enjoy!

Get everything setup and running then add "Cycle" to the tank per the directions on the bottle. Wait one week. This is minimum in my book - I would wait two weeks. Then consult your pet store or local fish store for advice on stocking the fish you want in the tank you have.