Firstly consider the definitions:
An elementary reaction is one that takes place in a single step, which does not involve the formation of any reaction intermediate.
Molecularity of an elementary reaction is a method of classifying the reaction in terms of the number of reactants species that take part
so for your reaction x----> products
it is an elementary reaction because it is a single step, no intermediate formed.
and the molecularity is one (unity) since there is only reactant species (x)
the rate law would then be:
v=k(x)
where k is the first order rate constant and (x) is the concentration of species x.
hope this is a bit clearer now