What resistor makes 10v into 5v?

You can't use one resistor to bring a voltage down, you have to use two in series in a voltage divider configuration, that is two resistors of the same value in serie between positive and negative, then you will have 5V between negative and the point between the resistors, any value can be used depending on the current jou are going to draw, note: you can not use to small a value at least 1K or up, if the value is to small the resistor will get to hot and it can burn out or it can burn your power supply or you can simply use a 7805 voltage regulator chip, it is inexpensive and easy to use, it only have 3 pins 1 = input, 2=GND and 3 is your output, one can get it in many deferent voltages 7803,7805, 7808, 7812 etc. Note: you can even adjust the output voltage between 5V and the RMS value of your power supply, by connecting a pot between GND and the center pin of the regulator, a smoothing cap is needed between GND and the input and one between GND and the output 1 to 10 mf 16V