edit: I really should refresh the page before I spend so much time repeating almost exactly what BobK had typed several hours previously...
edit2: Actually, I did add a bit more.
Komalbarun thank you very much God bless you,you are blessed..
All true..
Well it would have been if the advice were good (and it's not).
For powering a bulb you could use a simple series resistor. Beware that for a 5V globe running from 12V, the resistor will dissipate more power than the bulb. Assuming the bulb is a low power one this may not be a problem.
If you know the bulb's power or current requirements you can calculate the resistor you'd need.
Le's assume it's 1W.
The current is determined by P = V * I, so I = P/V = 1/5 = 0.2A
If you already know the current, you could skip that step.
Given a current of 0.2A, a bulb voltage of 5V and a power supply voltage of 12V, you want a resistor which will drop (12-5) = 7V at 0.2A.
Using V = I * R and rearranging to give us R = V/I, we calculate the resistor as 7/0.2 = 35 ohms.
You can't get a 35 ohm resistor, but you could use the next highest easily available value, 39 ohms.
Now we need to calculate the power dissipated in this resistor. This is given by P = I^2 * R (you can derive that from the two formulae given above). This si 0.2 * 0.2 * 39 = 1.56W
Because you want to leave some margin for safety (and so you don't burn your finger if you touch the resistor), you would select one with a power rating of at least 2W, but probably higher would be better, even a 5W one would not be too far over the top.
So, get a 39 ohm 5W resistor.
But remember, you'll need to repeat these calculations for the actual power or current requirement of your bulb.
Another option is to use a 5V voltage regulator. This is a little bit more expensive, but will allow you to run a load that varies from nothing to about 1A with no problems with changing voltage. This will require a 7805 regulator, preferably some small capacitors and almost certainly a heatsink.
If you really only need to run a single bulb, a resistor may be a viable alternative.