Noob question regarding DK

I know DK will be released 2 months later, this is not what I am asking.

The question is:
*when DK will be released, you will have to get a char to lvl 55 to turn him into DK? or you will have to create a new char?
*what if you have a char > lvl 55 and your turn him into a DK? you get a DK with your current level?

I didn't play wotlk on retail, sorry I don't know this stuff :D

You don't "turn" a character into a Death Knight.
You just have to have a level 55 character in Lordaeron to unlock the ability to create a character with the class.

Also DK is being released the same time as WOTLK... there is no extra wait on them.