If you plan to enter The Abyss and have any chance of surviving, it's an absolute must to have the Covenant of Artorias Ring, obtained by killing the Great Grey Wolf Sif in Darkroot Garden, equipped. For further details on how to reach the fog gate that leads to The Abyss, please refer to the New Londo Ruins walkthrough.
To reach fog gate that leads to the The Abyss you will have to have lowered the water level in New Londo Ruins. Full The Abyss Walkthrough Dive into The Abyss
