The trick is to find stats on your opponent (that are too high/too low) you can exploit.
A basic HUD will only be one line:
player's name/ VPIP / PFR / AGG / #hands
VPIP = % hands he plays
PFR = % hands he raises
AGG = aggression factor
See link above for 'color ranges' of other stats you can add to your HUD, for example: Fold BB to Steal... Fold to Continuation bet, etc.