Did I mention that I'm totally impressed that you're starting NOW, rather than saying "I'll wait until after Christmas" or "It'll be my New Years Resolution." It's obvious that you're serious about making this happen.
I don't think your portions sound overlarge at all.
About dropping the diet soda (good choice, btw), a drink that I like to sip on during the day is spearmint tea (no caffiene) with coconut milk. It's kind of sweet, and hot drinks help keep me focused when I'm stuck at a desk.