How much prep do you need help with?
The organizing the Christmas dinner, or all the way back to shopping and wrapping and planning the diary etc?
If the latter, there are a couple of websites which have good planners - Christmas Countdown is a 6 week plan (that just happens to start this week!!) which aims to get all the shopping and wrapping done; cards bought, written and sent; house cleaned; and food organized and cooked - while still having a modicum of sanity left by the end; or the Flylady (I don't have access at work so can't link) has a plan which pretends you are going on a cruise on 1 December so it all has to be done beforehand so you can arrive home on Christmas Eve to everything being ready.
If it's more by way of organising the dinner etc, a great tip would be to get a Good Housekeeping (November or December issue) or a Christmas magazine as they often have plans and schedules to work from. My main bits that may help are:
Decide on your menu - do you want traditional turkey dinner, or something different? Do you want all the sides or are there specific favourites you really want?
Decide do you want to do it all yourselves or buy it all in, or something in the middle like a pre-prepared stuffed bird and gravy and 1 special side dish but doing the potatoes, veg and starters yourselves.
Order early for anything you want to buy in.
Go through your menu and make a list of everything you need for the whole thing. Start buying the non-perishable bits early, a few things a week with the normal shopping and put them in a separate place for Christmas (between actual ingredients for the dinner and also treats like chocolates, mince pies, fizzy drinks or whatever you want as a family). Remember things like extra bin bags, toilet rolls and festive bits like crackers and paper napkins.
Prep what you can in advance. So making and freezing breadcrumbs, or even fully made stuffing. Do a big pot of soup for a wintery day, and freeze half for Christmas Day starter. That sort of thing.
On Christmas Eve, DH and I always prep the vegetables and potatoes, so that we don't have to do it the next day (we're out until late enough doing other visits). We wash, peel and slice things like potatoes, carrots, sprouts, cauliflower etc - and leave those soaking in water overnight. Parsnips and onions are also peeled and chopped but not left soaking - just an airtight container.
You can also set the table in advance if you won't be using it for breakfast. Or put the cutlery, glasses, serving dishes and spoons, salt and pepper etc, and any decorative touches all together on a tray to quickly set it when the time is right.
A turkey does need time to rest, after its cooked, so take it out an hour before you will be eating it and cover with tin foil and then a couple of folded bath-towels over the top to keep it hot. This means you then have the oven free to cook roast potatoes and veg, or starters etc, before you sit down.
And there are plenty of dishes that you can cook on the top of the hob if oven space is an issue - glazed carrots (with some olive oil, an onion and a very small amount of water), sprouts, mashed potatoes, plain peas, gravy etc.
And if things run behind time, have a few things that you can (all!) nibble on while you cook (not loads but enough to prevent a riot) - and relax and enjoy!