It's pretty straightforward getting to Prague's Old Town from the Ruzyne International Airport by taking a combination of bus and subway. It'll take about 30-40 minutes depending on traffic. The no. 119 bus runs from the airport to the Dejvicka metro (green/A line), from which it's four stops to Old Town's Mustek stop (a three-minute walk, according to Google Maps!). Make sure to pack some Czech crowns to purchase your bus/metro ticket from one of the kiosks at the airport or on the bus itself (latter is slightly more expensive).