Depends what kind of player, but generally the first board.
Speaking generally:
Try to find out as much info as possible about your opponent.
a) Stuff like PF calling frequency is important, if the guy is only calling with strong hands OOP (such as broadways, pairs, Ax, suited connectors maybe?) you can narrow their hand range down much easier than a guy that calls a lot more frequently.
b) How aggressive are they?  How bluffy are they?  If they are constantly bluffing and are very aggressive, you might not want to cbet the driest boards with all of your weak hands, especially if they have a turn leak (such as always leading with air on the turn when you check behind the flop but checking their weak to mid pairs, this means you can bluff raise these guys when they lead the turn very profitably).
c) What kind of hand do you have?  If you have Ace high and the player is rather passive it might be best to just play your position and check behind.
I should have probably added something to the beginning of this post about board textures as well, but simply put, try to line up their preflop calling hand range with the board to determine how likely it is that they hit or miss certain flops.  Once you get to that point the rest is a lot easier and will make much more sense.