Moving to Japan and working

I am a 23 year old American, just renewed my Japanese passport during the summer, and have never lived in Japan.

I am planning to move to Japan for a year next Spring and work. Since I have a Japanese passport, I don't envision any problems. The thing I'm most concerned with are documents necessary to work.

For example, in the US, you need a social security number to work. I don't have things like a juminhyo, etc. in Japan. I've never paid nenkin. Are there any things I should be careful of before I start looking for employment?

You must have a kosekitohon in Japan in order to get a Japanese passport. You don't need a social security but if you don't speak as natives do, the employer will ask a proof of the citizenship.

I already have a Japanese passport and read/write business-level Japanese so I wonder if it's safe to say that I wouldn't have any problems paperwork-wise working.

I'm just worried because the last thing I want is to get to Japan and then have to waste valuable time getting paperwork sorted out.