I am arriving in Shanghai from India on the 18th december.

To guarantee your trip, you'd better buy the ticket as early as possible since China Rail is the most popular transportation mode in China. It's not so difficult to buy it abroad. You can find a reliable website to book it online, like this website. It's quite easy to do so and they will give you a ticket pickup number after it is issued. You just need to use your passport and this pickup number to collect your paper ticket at the station before departure.