How do i find the first value every month

I have a table with a new value every hour:

Date Time Value

2005-01-01 00:00:00 1
2005-01-01 01:00:01 4
.. . .
.. . .
.. . .
2005-01-02 00:00:00 34
.. . .
.. . .
2005-02-01 00:00:00 145

etc..

The only rows i need from this table is the first value every month (date,
time and value), but I how do I do?

(timestamp can differ a couple of seconds every our)