Most important use for new players is to buy city Expansions with Medals.. It's an escalating scale you can find here:
That's weird the link doesn't work.
Anyway open up the build menu City View, lower left corner. Click on the Expansions tab. If you hover over the Medals Expansion it will tell you how many you need to buy the next one.
Please, please, please. Don't build Colosseum when you decide you want Medals. Message me or post in this thread and I'll give you a better strategy.