I'd like to say 'i'm currently working my notice and will start my new job in January. It's a step up for me: I'm moving into a management role.'
Sorry that it's quite long! I'm actually studying Japanese but much of this isn't in the grammar books I have!
I'm not sure how to translate "my notice".
I think you got a proposal about new job. So how about this?
Jirei o uke, ichigatu kara atarasii shigoto o hajimeru kotoni narimasita.
Watashi ni totteha suteppuappuni narimasu:
kanrishoku ni narimasu.
About English-Japanese translation, I think the service provided by Yahoo!Japan might be a better help than than Google's.