1) Go to
www.selloffvacations.com and search "All South" for the dates you want, and amenities (star level, beach-front, all inclusive, etc.)
2) Look at the available options and decide what you're comfortable spending.
3) Go to
www.tripadvisor.ca and search for reviews on all the hotels that meet your requirements.
4) Pick the hotel that has the best reviews and meets your wishlist.
Usually I'd say to wait until far closer to your travel date before booking to get a better deal but there's some pretty good deals in February 2014 right now if you're not going during Feb break.