Hook that photodiode up to your multimeter and see how much current it generates when you expose it to a candle flame. Bury it in some candle wax to simulate exactly what you would do for the final project.
Once you know how much current it generates you can design a proper circuit. You might start with something like the attached. (Note the current source, diode, 10M resistor and 10ohm resistor are simulating the photo diode.)
Throw that in a simulator and replace the 10uA with the current that the photodiode actually generates. If you can find a datasheet for the photodiode then you can plug in the proper parameters for the shunt and series resistance as well.
Once you do that it should just be a matter of playing around with the values of P1 and R3. These will be very dependent on the transistors' gains, however, so be sure to use the proper transistor part in your simulation. Even
then the actual transistor betas will vary so watch out for that. The values of P1 and R3 will also depend on the amount of current generated by the photodiode.
Also remember that, as your batteries are used, their voltage will droop so you'll want to simulate with a voltage of, say, 4V to make sure the circuit still behaves. If you are using coin cells then it might be wise to factor-in their series resistance, which is fairly high. (Something like 20ohms, if I remember correctly.)