Unlocking an iPhone. What do I do?

I was planning on going to a shop or market stall, somewhere where I can get an iPhone unlocked this weekend. The only thing is that I'm not the most intelligent individual when it comes to technology and so on.

The iPhone was previously used by my dad but he upgraded a few months ago, all I'm aware of is that it needs to be unlocked for myself to be able to use it. Do I need to reboot it or anything like that? I've also noticed that there's no way of signing out of the app store which means I'd be using his account instead of my own. Should I import any photos he wants keeping on to the computer due to the fact that I may need to reboot the phone or do I not need to?

Once I have had the phone unlocked will it be available to be used with any sim card, including my current sim which I'm using in my own contract phone?

Also, after its unlocked and if required rebooted, will it be completely unrestricted or will there be some features unavailable to use?

Oh, and finally… How much does it usually cost to have an iPhone unlocked, and will the guy/girl who unlocks it be searching through any private stuff on the phone?

I'm aware that some of these questions might sound quite stupid, but hey… Like I said, I'm not the most technical guy in the world.

Any help is appreciated!

Thanks,

Ben

Added (1). P.S I'm on a different network to my dad if that makes any difference?

Unless you are putting it on a different network than the one it is currently on, why unlock it?