How can a country ensure its national currency is used by its citizens?
First of all, please check the properties of money and keep them in mind. Indeed money is a convention, but nobody forces you to use it. For example, prisoners use ciggarettes as a medium of exchange.

But let's assume you are the absolute ruler of a country and you want your people to use your currency. You can either do it by force or you have to give incentives to your citizens.

The most important function of a new currency is to hold its value. If let's say my new currency holds value against yen, dollar or euro, then there's no reason for not using it. It's convenient and stable.

How to do this? Via monetary and fiscal policy, using gold standard or by giving it prefix value against another stable currency. For example, a bunch of eastern european countries have a prefixed exchange like Bulgaria (check Bulgarian lev in wikipedia).