A structure is the definition of a new datatype, not the definition of a new variable.
But you can do both in a single step:
struct some
{
int a;
float g;
struct some *n;
} mysome;

to define mysome as the above unnamed structure (which means you can't use that structure anywhere else).

By the way, "struct some n" won't work within the definition of struct some; you have to use pointers, because at that point "struct some" isn't fully defined, so it doesn't know how much space to allocate. But it knows how big a pointer is so that's no problem.