Hi Jodenice
I got back a couple of weeks ago from my first trip - it was wonderful

Make sure you get out & eat at the beach shacks - the food is fabulous & the staff will look after you really well. The Horizon at Calangute is good - best prawn biryani we have ever had, and the Travellers Rest at Baga is good too. You can spend all day at the shacks - the sun loungers are free as long as you buy the odd drink
You're not allowed to take rupees into or out of India. I took some sterling cash & sterling travellers cheques in small denominations (£50 & £20) because at around 84 rupees to the pound you can end up with a huge wad of notes if you're not careful! I spent less than £250 in two weeks - that included meals, drinks, taxis, overnight accommodation for a couple of nights in other areas, a 3 day trip to Hampi by train and most of the gifts I brought back

We ate at beach shacks for the most part, although we tended to have breakfast at the hotel as it was included.
We changed travellers cheques at our hotel as the exchange rate was exactly the same as we saw advertised elsewhere & this meant we didn't have to cart our passports around.
It was hot when we were there & I expect it'll be even hotter in Jan. Casual seems to be the rule & loose clothing is probably better as it helps keep you cooler. There's usually a nice sea breeze though which stops it becoming too oppressive. We were advised not to go much more than waist deep in the sea as there is a very strong undertow & it's easy to get dragged out. It's lovely & warm, though & it's great fun just riding the waves into shore - try not to graze your knees like my sis in law did!
Hope that's helpful & have a brilliant holiday