How to get 12v 5amps battery setup for LED strip
As Vladimir has stated, you'll definitely want much more power than a 9V battery can supply. You'll also want to ensure you don't waste lots of your energy through a resistor due to your supply voltage being much higher than your load voltage.

If you want a mobile light box, either find yourself a lead acid or lithium ion battery pack that will supply the amount of current you're after for extended periods of time. Your circuit itself isn't wrong besides the use of an 18V power supply to power a 12V load. At best, you'd have 66% efficiency (12/18).

AT 5 amps, you're going to need a heavy duty resistor. An ideal led supply would give it constant current, but that's likely too sophisticated for what you want to do. Given that these are power LEDs, their voltage-current curve isn't terribly steep and will likely be able to handle a bit more power so long as the power supply is relatively close to 12V. If it goes above 12.5-13 V on your supply. I would expect you to start decreasing the lifetime of the LEDs significantly. If you measure your supply (batteries) and it's below that range, then you would likely be able to eliminate the use of your resistor entirely. If your supply is above that range, then you'll want to determine how much you need to drop the voltage. Basically, you want it to drop 5 Amps down to 12 Volts.

For example: lets say your power supply gives 14 volts rather than 12 V. Go ahead and add a resistor that drops it to 12V. (14V-12V)/5A= 0.4 Ohms. That is the application of Ohm's Law. If you don't know or understand it, wikipedia is a great place to start.