Once you have constructed a database, you can access it by:
running the Postgres terminal monitor program (psql) which allows you to interactively enter, edit, and execute SQL commands.
writing a C program using the libpq subroutine library. This allows you to submit SQL commands from C and get answers and status messages back to your program. This interface is discussed further in the PostgreSQL Programmer's Guide.
psql dbnameYou will be greeted with the following message:
Welcome to psql, the PostgreSQL interactive terminal.
Type:  \copyright for distribution terms
       \h for help with SQL commands
       \? for help on internal slash commands
       \g or terminate with semicolon to execute query
       \q to quit
dbname=>
   This prompt indicates that the terminal monitor is listening to you and that you can type SQL queries into a workspace maintained by the terminal monitor. The psql program responds to escape codes that begin with the backslash character, "\". For example, you can get help on the syntax of various Postgres SQL commands by typing:
dbname=> \h
    
    Once  you  have finished entering your queries into the
    workspace, you can pass the contents of  the  workspace
    to the Postgres server by typing:
    dbname=> \g
    
    This  tells  the  server  to process the query.  If you
    terminate your query with a semicolon, the  backslash-g is  not
    necessary.   psql will automatically 
    process semicolon terminated queries.
    To read queries from a file,  instead  of
    entering them interactively, type:
    dbname=> \i filename
    
    To get out of psql and return to Unix, type
    dbname=> \q
    
    and  psql  will  quit  and  return  
    you to your command shell. (For more escape codes, type
    backslash-h at  the  monitor prompt.)
    White  space  (i.e.,  spaces, tabs and newlines) may be
    used freely in SQL queries.  
    Single-line comments  are  denoted  by two dashes
    ("--").   Everything  after the dashes up to the end of the
    line is ignored. Multiple-line comments, and comments within a line,
    are denoted by "/* ... */", a convention borrowed
    from Ingres.