In plain RDF, you can simply do this:
my:relation a my:SymmetricRelation .
This expresses that the relation my:relation
is of type my:SymmetricRelation
.
However, if you also want a reasoner (such as an OWL reasoner) to do "clever things" with the fact that your relation is symmetric, then plain RDF is not really enough, and you'll need to switch to OWL, and model your relation as an owl:SymmetricProperty
.
my:relation a owl:SymmetricProperty .