ohms law

What are you trying to accomplish? It sounds like you need a voltage regulator, not a voltage divider.

do you want do create voltage divider to get 5V  from 10V?  What level of current are you going to draw? As baldengineer have stated, using divider would be really wasteful - regulator would be better choice

using circuit as above, with two resistors of the same value, you can get half of voltage in the middle point - if you draw only a small current (if you draw more, voltage across R2 drops)

what i am trying to say is how to choose a resistor for a led all tutorial i have watch seams they get their current from datasheet and they use ohms low to work out and get the suitable voltage for the led and i have been founding it difficult choose a resistor for my circuit because using ohms low is voltage divide by current and i am using batter so what should i do

This is a community created online calculator.

Voltage divider calculator for real E-series resistors (douxchamps.net)

for driving a LED you need three values: power supply voltage, diode forward voltage and diode current.

Diode current can usually be in the range of about 2-20mA

Diode voltage depend on the color and is standard for that color - for example, yellow LED has forward voltage of about 2.1V

then, all the remaining voltage should be across the resistor - so (for example, using 9V battery and 5mA)

9V - 2.1V (diode forward voltage) = 6.9V (voltage drop across resistor)

6.9V/5mA = 1380 Ohm

this is not a standard value, but you can use - for example - 1.3kOhm (1300Ohm)

what about if i want to drive a 12volt motor and i have a voltage supply to be 18v what should i do

You should get a basic multimeter, and a small pack of resistors, and just try it. Take measurements, observe if anything gets hot, and compare with your expectations. What's the worst that will happen? Combined with some reading, I guarantee you'll learn more that way than the mistakes you make.

Ohm's law is fundamental, and will be covered in many books, but there are excellent recommendations here:

 I Want to Do Electronics!  Where do I start?

install a voltage regulator - switching one if possible to limit power loss.

Motor has a different characteristics - it consumes different current depending on the load. You can probably limit the current using resistor but it would be wasteful - if you calculate the resistor for idle current, any load increase would increase voltage across resistor, decreasing voltage across motor and limiting power. And if you calculate resistor for loaded motor, you will get overvoltage when motor is idle.

Anyway - if you can tell what current   will be drawn during operation (for example - steady load), the method is the same - all the excess voltage should be across resistor.

for example - motor consumes 2A during operation, and voltage difference is 18V-12V = 6V

6V/2A = 3 Ohm

and power dissipated would be 6V*2A = 12W

and of course - if motor is stalled, most of the input voltage would be across the resistor, resulting in 18V * 18V/3Ohm = 108W of power

The question is not clear to me.
Assuming you have a resistor with 10 volts, connecting an identical resistor from that resistor to ground will drop the voltage to 5 volts.