how can I unlock my n73 ?

If you are referring to unlocking it from a network, you can't.

Only the network provider can do that and they will normally charge a fee, especially if the phone is still under contract.

Do not believe anyone who says they can do it. They can't. Simple as that :)

I've heard a T-mobile sales assistant saying that all the networks will unlock your phone for free, he seemed to think it was something to do with competition laws, but I've never experienced this first hand.

It might be worth phoning your network operator and asking.