It depends how far you want to go really doesn't it? You could follow a whole diet and fitness regime (eg Body Coach), in which case the balance of fat/carbs etc is worked out for you.
But in terms of just eating more healthily in general...
I love salmon with couscous and roasted Med vegetables. Really fast meal and loaded with goodness.
I try to make sure if I'm having a "meat and 2 veg" type meal that half my plate is veg, and not too many carbs.
You could cut back on meat dishes (not inherently unhealthy, but can make you more mindful in general about what you're cooking).
What exercise will you be doing? Do you exercise normally? If not, I'd really recommend couch to 5k, and sign up for a run at the end of it (or a Parkrun) to keep you going to graduation. Or Yoga with Adrienne does a 30 days of yoga every January which is lovely. And I also really like Fitness Blender for home workouts, being able to choose length/intensity etc to fit in with the time I have available is great.
But my best tip, if available to you, is to get some sessions in with a personal trainer, even if it's just once a month. They'll teach you good form, lots of different exercises that you'll be able to do yourself, and they'll keep pushing you (plus it keeps you accountable in between sessions - no point having a PT if you don't keep it up in between, plus the FEAR of going to a PT without having done anything in between is like nothing else lol! The pain!).
Good luck! Another thing I'd say, is why wait til January? I know December is a crazy month, but I find if I can fit some workouts in while I'm off work it helps with all the rich food etc (DH and I try to do a short jog on Christmas morning after presents and while food cooking. Makes us feel very worthy haha).