How to create a spatial index on point data.

How does once create a spatial index on point data. I did the
following and got the error given below.

create table foo(x point);
create index foo_x on foo using gist(x);
ERROR:  data type point has no default operator class for access method "gist"
HINT:  You must specify an operator class for the index or define a
default operator class for the data type.

Thanks.