A lot of good advice here. Bottled water, avoid fresh fruit unless you're peeling it yourself, avoid raw veggies,fruit juice if not from a sealed container.
Hygene standards, especially at restaurants & hotels catering to the foreign tourists have come a long way in the last couple of decades, but with a short visit, it is best to take the precautions listed above. Eat in places with a lot of turnover - and eat hot (temperature) food.
Mosquitoes shouldn't be a problem in January, but take & use deet mosquito repellent and cover your skin - long sleeve, long trousers, socks & shoes. If you can, treat some of your clothes with permethrin before you go. Take an anti-malaria tablet like Malarone (
https://www.cdc.gov/malaria/travelers/drugs.html)
Ask your doc for a dose of antibiotics to treat any intestinal bacterial infection you may pick up. Our Kaiser Permanente travel clinic provides us with one & we used it on one trip.
By "Golden Triangle" I assume you're going to Delhi, Jaipur and Agra. The sights to see there are well known. For the Taj - try to get there before dawn. They open the gates at sunrise and the taj is magical at that time. Hopefully, there will be no fog when you go. In general, try to go early to sites. If you have the time while in Agra - visit Fathepur Sikri & Akbar's tomb. In Delhi, one of my faves is Himayun's tomb. A lesser known gem is Safdarjang's tomb. India has a new parliament building & the old Rajpath (now renamed Karthavya path) has been redone. I haven't been there yet so can't tell you what to expect.
Uber works well in India, but we saved a lot of time in Delhi by using the metro. The local trains in Mumbai are usually very crowded, but the new monorail may not be (I haven't used the latter).
In Mumbai, a sunrise walk along Marine Drive is wonderful. If you're in the area, there used to be a superb Konkan restaurant in the Taj President at Cuffe Parade. I love walking around in that part of mumbai -Fort, Marine Drive, Gateway of India.
I've been in the US since 1986, but go back to visit every few years. Early 2020 was my last visit.
India is fascinating. Embrace the difference. Enjoy yourself.
The Taj at dawn: