You'll have to improve your knowledge then.
The resistor is on the IR LED to keep from destroying it. It limits current.
Get the data sheet for the IR LED and find the forward voltage, use +5 for the source voltage since it'll be running off the regulated arduino supply.
The pick the closest higher resistance. 220 ohm works for standard ones but the IR one may vary.
I just found an instructable
Since you want a beam break you don't need the IR LEDs on the digital pins. They can go to +5v.
They don't show it but the text tells it that +5 goes to pin 2 and the 10k resistor then the other side of the resistor goes to the photo transistor. The resistor is pulling up pin2 and the photo transistor grounds it. You'll be looking for the digital pin to go low.
Ill see if I can get my head round that.
What resistor would i use and where would i put it. My knowledge of electronics is very limited!
Also, how would I declare a phottransistor in the Arduino environment software?