PostgreSQL. What is the best way to create indexes on a table?

Question

PostgreSQL

There is such a table:

CREATE TABLE tbl
(
  f1 bigint,
  f2 bigint,
  f3 bigint,
  f4 bigint,
  f5 bigint,
  date timestamp with time zone,
  value double precision
)

From it, the selection of the field value with group by on the fields f1 -f5, date. + can be a condition in where to filter the data. The set of fields in group by and where can be arbitrary.
How it is better to create indexes to such table?