As in any city, the best way to enjoy the city centre is simply walking around. In fact the city centre is not that big and you can enjoy long walks from Prague castle to the old town square, passing by Mala Strana and Charles Bridge. It is a very enjoyable walk and you won’t get lost… you just have to follow the flow of people!
Still if you’re short on time, or don’t feel like walking too much, Prague public transports are very good and quite cheap, and taxis are not expensive either. Should you get a taxi for a short inner city ride, you’ll probably end up paying something around 5 euros.