How do I figure out the exact voltage and current needed for a part?

Can explain me in details please?

Well, of course, if you have a source and you know the resistance of that part, you can get the voltage and current passing through that component. But generally, parts have datasheets that lets you know the exact configurations of what its capacity to handle is. In other terms the operating voltage and current.

As Kevin said, it's normally the other way around, you choose a part to give a desired voltage or current, which comes out of the design process. You then have to choose the part that will meet and exceed those specifications, for example, the maximum voltage, current, or power rating that the part can cope with. That information is in the data sheets.
You normally would never look for an exact spec, you need to have a margin, to prolong the life of the part, for example.
If your question is being misinterpreted, the re ask it with an example, or a bit more detail,
cheers,
Richard Gabric.

Mark... Where to begin?  A bit more detail on what you are trying to do would be good.  Lets start with something simple.  you want to operate a LED at 20ma from a 24V supply.  so you need a dropping resistor.  the LED will take 2V so the resistor will have to take 22 V.  From Ohms law, 22V at 20mA will need a resistor R = V/I = 22/0.02 = 1100 ohms.  Here's where the selection comes in.  you could use a 1000 ohm resistor, which would give a bit over 20mA LED current, and might cause the LED to fail early.  Or you could use a 1200 ohm resistor, which would give a bit less than 20 mA and the LED would be less likely to fail.  if you do that, then work out the power dropped in the resistor:V2 / R = (22*22) / 1200 = 0.4 watts approximately.  I'd use at least a half-watt resistor, rather a 1 watt if I could get my hands on one.Another example - lets say you're making a basic 12V power supply with a transformer, bridge rectifier and capacitor.  If your transformer puts out 12V AC, when you rectify it the peaks will be 1.4.4 x 12 = 16.9 volts.  So your capacitor should be rated for that voltage AT LEAST.  I would use a 25V capacitor as a minimum.  If your supply is going to put out 1 amp, use 2 amp diodes.  and make sure your transformer is rated to supply this current.All you need in most cases is Ohms Law, plus a bit of guesswork, always make sure your component can take more voltage / current / power than it needs to.  Rating the component to take double the value it will actually have is good practice if you can.  
I hope this make sense.  If not, I'd suggest you study basic electricity concepts more before you try this sort of stuff, not trying to talk down to you here but there's no indication of what you know and what you don't.... good luck. / David