Before you add fish to your new aquarium, make sure that the tank is fully cycled.   You can read instructions about this here.

To add fish to an existing set-up, it is a good idea to quarantine them for a couple of weeks or so in a separate tank, to make sure that they are healthy.  To do this, you need to keep a spare smaller tank set up and cycled ready.

Fish must always be acclimatised before going into any tank.  Read about this here.