Hi Nick - Wish I was arriving in Calangute on Sunday!! Irishcolleen's advice sounds good. If you can spend the first day or so in a shack that's recommended, it gives you some time to get the hang of the place before you spread your wings. I always find it difficult recommending shacks because it's all about how you get on with the people who operate them - we're all different and you'll find your own favourite.
As for the first meal, it depends on what you want. You're not far away from After 7, which is a fantastic restaurant although for Goa, at the more expensive end and quite swish. The Plantain Leaf in the centre of Calangute is the other extreme (basic but very clean and very cheap) and the vegetarian Thali de Luxe (around £1) in there is wonderful.

It may do you on your first night when you'll be tired anyway. Suza Lobo is by the beach and good for seafood.
Have a great holiday!