This might be too late to help you, but who knows ...
Normally, LEDs will be wired in series with a resistor to adjust the voltage across the LED quite closely and to provide a crude form of current control. LEDs are very sensitive to voltage changes and quite small changes will cause the current either to drop uselessly low or dangerously high. A resistor makes the current less wildly sensitive to small voltage changes and protects to some extent against surges and small voltage drops you might get from long wiring runs or old batteries. There are more sophisticated ways to control the current through the LED but not something I could design and probably not something you'd want to wire yourself.
So that said, you might not need a resistor at all, depending on how you're driving the LEDs. If you have a suitable power supply, most likely intended as an LED driver and probably tagged as "constant current", you might not need the resistor because the driver will moderate the forward current for you. Note that a single constant current driver may not be sufficient to properly regulate multiple LEDs is parallel and you may still want a resistor. Some LED drivers are designed to regulate the current to multiple parallel loads.
Still, assuming you have a more crude 12v power supply, you need to decide how hard you want to drive the LEDs. You've said 280ma, but that's probably just a package spec. You should have a wide range of possible currents you could use depending how much light you need and how long you want the LEDs to last. Let's go with 280mA anyway. You should also find out the forward voltage at 280mA, lets say it is 10V. Now you know you want 2V across your resistor at 280mA, so that would be about 7 ohms. Sounds tiny to me, but I'm used to working with lower voltage and lower current LEDs. There are online calculators to do all the sums for you if you're not confident with this. Try
http://ledcalc.com/
The power supply rating again depends how hard you're going to run the LEDs. You know the voltage is 12V and you pick an operating current, let's say 280mA per LED. Multiply by 9 LEDs, add 20% for losses and overhead, that gives you 3 amps. Your 2.5A supply would most likely light up the bulbs and you'd think you were good to go, but it would probably overheat and if you were lucky fail. Unlucky and it would burn your house down
