About when we should setup index?

Hello,

I have some questions about index.

For example, we have a table
test(id varchar(7) not null primary key,
name varchar(15) ,
sex varchar(1) ,
birth_date date,
valid_in_table boolean not null,
time_stamp timestamp not null default now() );

. id is the primary key, so a default unique index is generated
automatically ?
. if queries based on name are often, index should be setup for name?
if there are 30,000 records, 29,000 records' names are different, will
the index for name still be useful?

. possible values for sex are F/M and null, should we setup index for sex?

. How about index for date and timestamp?

Basically, I'd like to know is there a percentage of the differences
among data to decide whether index will help or not? For example, among
30,000 records, for a column, its value choices are less than A% and
greater than B% so that we know index will help a lot?

thanks a lot!
Emi