First of all, DO NOT exchange money on the street, even if it looks like that he just pulled it out of an ATM!
Most of them are outdated Belarusian rubles, which are no longer valid even in Belarus, let alone in the Czech Republic.
The official currency of the Czech Republic is the Czech crowns (Česká koruna).
You can usually pay with a foreign card, but it's always a good idea to have at least a few crowns in cash.
You can exchange at the exchange office, but remember:
1 EUR = from 21 to 26 CZK
1 USD = from 20 to 25 CZK
If you are not sure, just write 1 EUR to CZK into Google.