There are many ways to unlock an LG Arena, a lot possibly depends on where you are living in terms of what is available to you.
I wouldn't recommend attempting it yourself, as it does require some technical knowledge. Luckily, there should be quite a few people able to help...
The first place I would start would be with the network that the phone is locked to. They will normally unlock phones, perhaps for a small fee, and at least you know that if they unlocked it you are covered if anything goes wrong and it should not void your warranty.UK networks are being 'kinder' to us mobile users now, I know that the 'Three' network sells all phones unlocked, and if a contract is with o2 you can request an unlock code from them immediately (on pay as you go you need to pay a fee or wait a year though).
There are also websites that offer unlock codes with a money back guarantee and I think I am right in saying that these 'shouldn't' mess up your phone if you follow the instructions. Be aware of free ones and downloading software to your computer as these often contain malware.
Yet another option is to take your phone to a shop (in fact in the UK many market stalls carry out the service too) that unlocks phones, they should be able to do it while you wait.
I hope that helps.