You can buy your tickets directly on the trams: there is a yellow machine in the middle area of the trams. There, you can purchase tickets with your card.
If you would like to buy paper tickets with cash, you may purchase them at any kiosk or mini market. The nearest mini market to your apartment is here:

Transportation tickets in Prague are valid for trams, buses, and metros in Prague.

Prices are:
30 CZK for 30 minutes (adult)
40 CZK for 90 minutes
120 CZK for 24 hrs
330 CZK for 72 hrs