To be honest, I don’t think about it too much these days. International phone and data roaming are a lot cheaper than they used to be. That said a) you have to make sure they’re turned on, and b) they’re still not that cheap.
So, if you don’t have much experience traveling abroad, make sure that before you leave, you contact your mobile carrier and select an appropriate international calling plan. It’s probably not the end of the world if you don’t have it, but if you’re traveling abroad for any substantial period of time, you’ll want to be sure you can receive any critical calls from home, and you’ll want to be sure you can at least call someone in case of emergency.
There are lots of places with free WiFi in Prague, so international data probably isn’t essential. There’s limited free WiFi at the airport when you arrive, and at many of the tram stations and most cafes around town. That said, there’s “essential” and then there’s “nice”. If your carrier provides a reasonable plan, you should seriously consider enabling international data roaming. Even if it’s a fairly limited plan (in which case, you can turn off mobile data roaming on your phone, and only switch it on in an emergency) it’s probably worth getting.
Possibly the cheapest and best option is to pick up a local SIM when you arrive. There’s a Vodafone store in the airport that’s open from 9AM – 9PM, so as long as you arrive at a reasonable hour, you can pop on over and pick up a SIM. The store is located in Terminal 2, which is domestic, outside of security. If you’re flying from or have made a connection in the EU, that’s where you’ll arrive; but if Prague is your first landing inside the EU, you’ll end up at Terminal 1, and need to walk over (about 10 minutes). This is probably critical if you’re taking Uber from the airport; otherwise, you can pick up a data SIM from anywhere in town.
I generally find the best thing to do is to bring an older, spare phone, and put your home SIM (which you’ll hopefully rarely use) in that, and put your Czech data SIM in your day-to-day phone. But, no matter what, if you come from a country that still has the concept of “locked” phones, make sure yours is unlocked.
You can find current pricing for Vodafone Data SIMs on their website.