I try to get the sand/redsand from the composter,but I get the dirt.I am no sure is the sand only get at a time from a wandering trader?

You most likely have to be in a desertish biome to get sand or in a mesa biome to get redsand.

@QPCrummer is correct. It's based on the default surface block of the biome the composter is in. So in sandy biomes like deserts and beaches you'll get sand and in badlands type biomes you'll get red sand.