Slow response to my query

I have a HP DL380 server  g8 with 32GB RAM and 48 *2.5Ghz CPU. Running on
CentOS Linux 7.4.

I have a postgresql 12 database. One of my tables has 42 columns and has
about 500million records.
However, any attempt to select even 1 column from the table take hours at
least 1.5hours to respond.

Kindly help on documentation on how to optimize it to return the response
in milliseconds.

Regards