My reading of that Amazon page is that device is meant to be supplied with 12v - whether it has built-in current-limiting resistors, or something more complicated, isn't obvious.
So in your diagram, R2 isn't necessary
From the data-sheet for the transistor, the current gain at 300mA collector current would be somewhere between 30 and 100.
That means the base current will need to be in the region of 10mA
With the Pi's GPIO giving 3.3v, and the transistor's base-emitter voltage around 0.7v, there'll be ~2.6v across R1
At 10ma and 2.6v, R1 should be 260Ω. Something like 200Ω or 220Ω would work.
...I may need a diode to prevent current from flowing backwards...
That's recommended when you're driving a coil, or similar - something that stores energy in a magnetic field, or in a moving mass such as a motor
When the current through a coil is switched off, the energy has to go somewhere. Without the diode, it usually goes where you don't want it, and destroys something.
The diode allows current to keep flowing, and wastes the energy as heat.
You're not storing energy, so you don't need a diode.
