How to use my 3 3G sim in my iphone 4?

I have a 3 3G sim on contract. The sim is contract the phone was bought separately. I have recently been given an iphone 4 from a family member however, they were using an orange sim card in it. I was wondering if anyone could tell me how i would go about using this. Obviously i'll have to go into the three store and get a micro-sim as the larger one won't work with the iphone but other than that what will i need to do?

First of all we need you to buy a micro sim cutter and cut it to the micro sim. If you visit the stores, there will also use this cutter and cut it for you and charge a fee.

After doing that, you need to check if the iPhone is lock to the network, Orange in this case. If you plug in and it work then you are ready to go. If it is lock, then you need to spend money to unlock again. Next is to reset the iPhone and it will be like factory default. Lastly of course you need to setup an Apple account if you do not have one.

Enjoy your iPhone.

1 - get the orange iPhone unlocked to any Network
All iPhones lock to the network of 1st SIM that is put in it.
So call up Orange, pay the 15-20 pounds 'apple' charge to unlock
takes about 2 days usually

2 - go to Three, ask for a replacement micro SIM for the normal/mini SIM you have
so not try to cut it down.
The edges on a real one are finished so they don't stick in the slot.

3 - put the new SIM in the iPhone and it will complete the unlock process

FYI
You can get micro to mini SIM converters, but they can let the micro sim drop out and get trapped in some phones - so beware.
Best solution is to keep old and new SIM's and if needed go to the network SIM Swap page and change the numbers over yourself (most networks let you do that)