How to use resistors?

I have a power supply of 5 volts @ 1A. I want to reduce the current from 1A to 250mA. Which resistor should be use?

I tried this equation(Ohm's Law):

R = V / I = 5/(0.75) = 6.66 Ohms.

I tried the 6.66 Ohm resistor but it only reduce few mili Amps.

Answer

The 1A rating on the power supply is simply it's maximum, not the actual amount of current drawn by whatever you hook up to it. That is the amount you must ensure the load stays below.

If the "load" is a short circuit, then your calculation is nearly right. $$ R = {{V}\over{I}} = {{5V}\over{250mA}} = 20\Omega$$ If the load is something else, a resistor of lesser resistance is required.