Hi! Welcome to the forum.
First of all, you'll need to have water conditioner (dechlorinator). This will take chlorine and other heavy metals out of the water to make it safe. Every pet store should carry this.
Secondly, make sure you have a water test kit. The API Master Test Kit is the best, as it is the most accurate. You can use strips as well, but these aren't always as accurate. Whichever kind of test kit you get will allow you to monitor the levels of ammonia, nitrite, and nitrate in the water. This is crucial to getting the tank to cycle and keeping the levels safe for fish. If you haven't read about the nitrogen cycle, that should be the first thing you do. I can post a link if you need one.
Lastly, make sure that you have a good filter that is adequate for your tank. Keep up on your water changes to keep the water quality good. A tip is to use the conditioner on the new water you are adding before it goes into the tank.
If you have any more questions, I'm happy to help!