you will need to know the forward voltage and forward current of the LEDs. If this is unknown, and they are not superbright LEDs, its safe to assume 2.2V forward voltage, and a 20mA forward current.
basically you will be able to connect them in series, and have a resistor in series too, then connected to 12V.
so the basic setup will be :-
To work out the required resistor - the basic equation is Vsupply - Vled / Iled
in this case, because we have 4 LED's the equation will be :-
Vsupply - Vled1 - Vled2 -Vled3 - Vled4
Iled (in Amps)
(because the LEDs are in series, the current will be the same through each one. If you have different current LED's, you ALWAYS pick the lowest current for this equation)
in your case (assuming 2.2 Forward voltage, and 20mA forward current (or 0.020A)
12V - 2.2v - 2.2v - 2.2v - 2.2v
= 3.2V / 0.020
so you would need a 160ohm resistor in that case. if the LED forward voltage or forward current is different, you need to do the sums again with the new values. LED's usually specify a MAX forward current, and its good practice to go under this (in case of spikes etc) so if the MAX is 30mA, set the circuit at 25mA, to give some headroom. sometimes you can not get the exact resistor - its better going up than down in value, as this brings the current down a little. sometimes you can get away with going slightly down in value. :-)
If you connect without a limiting resistor, there is a good chance it will pop the LED's.
Also you need to make sure you wire the LED's the right way. Anode goes towards the positive, cathode to the negative. Most round case LEDS have a "flat-spot" nearest the cathode lead on the case. (see here
As for finding the 12v and 0V in the PSU, just follow the yellow (12V) and black (0V) wires. :-) A multimeter will confirm the 12V supply.
hope that helps.