Here is the questions:
1) How do i determine what minimum specification is required to power up a single Subwoofer (e.g. 1500W)?
- depends on specs of amp or specs of sub, which ever u get first, in watts rms, not max


2) How can i bridge a 2 Ch Amp, as in which + and - teminal to choose? (e.g. 1st Ch (-) with 2nd Ch (+) or vice versa)?
if the sub is single voice coil then its straightforward, +ve from one channel and -ve from the other channel. if its double voice coil then theres parallel or series connection, which u can google for diagrams


3) What will happen if the amp specifications is LOWER or HIGHER than the minumum requirements to power up the subwoofer?
preferably look for something 50% more than sub's rated rms power, look at amp's bridged mono power rms specs.
more importantly is to keep the amp's gain level below distortion level