A comma follows a prepositional phrase when it appears at the beginning of a sentence. The subject of the sentence immediately follows the prepositional phrase.
about
above
according to
across
across from
after
against
along
among
around
at
because of
before
behind
below
beneath
beside
between
beyond
by
despite
during
except for
for
from
in
inside of
instead of
near
of
on
outside
over
prior to
regarding
since
through
throughout
toward
under
until
upon
with
within
without