@ladyjane - Fresh spearmint leaves are what's in the traditional mojitos recipe along with guarapo or simple syrup and clear rum like Havana Club, which is preferred but a couple of good substitutes are Bacardi Superior Light or Captain Morgan's Silver.
If you can't find the sugar cane juice in your area then you can create a mojito simple syrup by heating equal parts of water and sugar. Keep Stirring the mixture constantly until the sugar is completely dissolved (no crunches in mojitos) but DO NOT let it come to a boil.
Once the mixture is ready throw in a handful of fresh spearmint leaves and let the flavors fuse together for about an hour or so. Strain the leaves from the syrup and then it's ready for use.