Could someone help me choose the value of the elements in this key? I am total begginer and I don't know how to start with it.
No, because that circuit is incapable of doing what you want. Q1 starts to turn on when its Vbe reaches about 0.6V. Until that point its emitter is held at 0V by R3. It then acts as an emitter follower, with a voltage drop from the input dependent on its load current, gain and the value of R1. Once Q2 Vbe reaches about 0.6V it starts to turn on and depending on its gain and the value of its base resistor R2, increasing the input voltage may cause it to draw enough current to short out the 5V supply. As a whole it doesn't have a 3.3V switching threshold and as far as it works (which wont be very well) has the opposite action to that you desire.
I want the 5V power supply to turn on when the voltage on the battery will be below 3.3V.
To meet your stated objective, you need some sort of a comparator circuit with a 3.3V threshold, driving a transistor as a switch, in series with the 5V supply, (or possibly in series with the ground to your load if its acceptable to leave its supply connected all the time and switch the ground). How to implement that depends on the parts you have available or can easily obtain, the accuracy required (and whether or not it needs a 'snap' action), the quiescent current you can afford for the circuit's operation, and the maximum load current.
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