This page of The Witcher 3 Wild Hunt explains where you can exchange coins (different currencies). This is important as only one of them can be used in transactions.
There are three currencies available in Witcher 3. The main and commonly used currency are crowns. Additionally, Geralt can receive or find in the game world some florens and orens. You can't use those coins at the beginning of the game and they are stocked in the inventory with standard items.
Luckily, florens and orens aren't useless. They can be exchanged for crowns by visiting a money-changer. Vimme Vivaldi offers such service and he can be found on the Free City of Novigrad. You will find him in the southwest corner of the Hierarch Square (M6,5 on Novigrad's map in our world atlas), not far from Triss Merigold house.
After initiating a conversation with Vivaldi you can ask him to convert some currency. The process is completely automatic, florens and orens are taken from your inventory and changed to crowns. You should return to Vivaldi regularly and change the gold, it will help you limit the trouble with the lack of gold, especially at the beginning of the game.
Important - In addition to exchanging money, Vimme Vivaldi can give Geralt a onetime loan of 100 crowns. As Yennefer is financing this loan, you don't have to worry about paying it.