Not really complicate.
It's like you buy a box of PC games. You found a code in the box.
Login to your PSN account (from your PS3, PSP or PC via Media GO software).
Input your code there and you can login to the online mode with your PSN account.
Easy than it sounds
One thing I like about online mode is; when you create a new character, your friend will still see your online status even he/she never meet your new char before.
But you have to meet each other and exchanged partner card at least once.
To say, the partner card tied to your account not character. So, it's easier to keep track of your friend.
Also, the game use common box now. So, items in the box can be shared to your other characters in the same account.
My character is level 1
I played a lot of Battle mode and Challange mode. Due to no level restriction.
And don't worry, everyone start at the same lowest level and so-so equipments to make it fair and square.