Ooh lovely - have fun. Ascot in the winter will be a real mixed bag in terms of clothing and dressiness, you will likely see anything from casual families dressed much as they would be for an outing to the park (jeans, waterproofs and wellies etc) through classic 'country' types in weather beaten tweeds, barbours and flat caps, youngsters under the mistaken opinion they're off clubbing in cocktail dresses and stilettos (plus goose pimpled flesh!), young men looking like something out of an episode of peaky blinders, all the way up to very very stylish couples in smart suits/dresses, wool coats, fur hats or trimmed trilbies - think the royal family at a winter wedding type of vibe. So dress up as much or as little as you like, if you're hoping to spend plenty of time in the paddock or at the rails to watch the races I'd certainly recommend a warm coat and waterproof shoes (you won't need to go anywhere muddy, Ascot is well set up in that regard, but if it belts down with rain you'll find out about any holes in your shoes!).
Other tips - Ascot is very well run and can feed and water an enormous number of people efficiently (plus Saturday wouldn't be their busiest day of the year) but I do find the food somewhat overpriced and middling quality, on busier days you will have to queue so budget that in for the day in terms of £ and time - I'd be tempted to have a big brunch before going and then maybe just get a snack on the racecourse - every single time DH ends up queuing for a £15 burger and complaining about it!
If you'd like a flutter I think it's much more fun to do so in the betting ring with the 'rails' independent bookmakers than with the Tote as then you can shop around for the best price but that does mean venturing outside again plus you should bring cash (some take card but not all).
Finally if you or the people with you have any questions about horse welfare or hear or see anything that concerns you about the horses and how they are looked after while you are at the races, this is a really good resource https://www.horsepwr.co.uk/ . Every time I go to the races and earwig people's conversations (something I do enjoy doing!) I hear silly, silly myths being spoken about as though they're fact. Things that would be really concerning if you believed them (e.g. any horses that finish the race last or are too slow get shot/taken to the abattoir/made into glue - no, these are very valuable and much loved animals, it would be incredibly stupid to shoot them after one bad race!), lots of misconceptions about the whip too, or how the horses are cared for off the racecourse. Without a doubt there are some welfare issues in racing but even as a casual racegoer it's good to be clued up on the real facts.
Enjoy!