Every machine is different...
but every one I've had has had a line at the bottom of the first POST screen which invites you to do something. Fr e.g. my present m/c has the following:
TAB for POST details, ESC for Boot Screen, DEL for Setup.
I seem to remember being invited to press F12 on another machine, too - function and computer type escape me now.
So, try some or all of the above, and see what comes up... alternatively there might be a FM somewhere for you to RT (You know, the one that you put somewhere safe in case you need it sometime)