I don't think you need to know loads - I didn't before I got the job I'm currently in. I had briefly run a VERY small Adwords campaign before and so I had a little bit of an idea of how it worked.
I think most important is attention to detail and being good with numbers and statistics.
You should know how the auctions work and things like - what would influence how much you would bid for a keyword.
There's loads to read here at the Adwords Learning Centre:
Google AdWords: Learning Centre