I bought a 2nd hand nokia 7373 and when i put sim...

To unlock your nokia you will need to download and upgrade the firmware. To do this you need to buy a computer connection cable, an unlocking box box, the software to connect to pc and to find the unlocked firmware somewere on the web.The whole equipment is far more expensive then the phone itself, and this procedure always involve a small risk to damage the phone. Here it shows the howto.
Here are boxes for sale

There are codes to unlock phones distributed by big providers (eg Vodaphone, Orange, Tmobile etc.) by inserting code, but thisinformation is usually not provided to users. To get code you should call the provider who branded the phone, and probably they won't give you the code for commercial reasons.
See here

If you have no practice with this, and you are not planning to do unlocking on more than one phone it could be convenient to pay the small fee required by phone shop, or use a mail unlock service like this :
Unlock Nokia N95 8GB