There is special paint that has a sandy texture. You can get it in brush paint or spray paint. I've used the spray paint and it looks awesome!
You can put on a coat of paint in sections, while it is wet add sand by tipping pot to one side and sprinkling sand on to the wet paint. Do small sections. When dry add another coat of paint to keep the sand in place. If you add the sand to the paint you have to constantly stir the paint to keep it from settling and don't get an even amount where you want it.