After tom riddle talks to you, you will be versing the basilisk. The basilisk is quite hard to beat so thats why i'm telling you how to defeat it. So this is how...

When the basilisk comes through the drain pipes, you use your sword to blast it. You will find the basilisk lays down, then shows its belly. There is a little mark which is a diamond with a dot in it on its belly. When you see this, that is when you blast it again.

The blast stays in the air whilst amost hitting the basilisk and you have to move it around with your analog stick or your arrow keys. When you hit the basilisk your sword will fly our of your hands. Then you must find your sword and keep repeating this blasting process untill the basilisk is defeated.