The giant gate through the Undead Parish leads to an area called Sen's Fortress. This is the major path to advancing the main storyline. Your level is in the right area for heading this route, so you won't have any more problems than normal.
After Quelaag, the next area is the Demon Ruins and it requires you to fight a boss called the Ceaseless Discharge. Minor tip about this fight:
The boss doesn't permanantly aggro until you take an item at the end of the area. I suggest not taking it until afterwards, so you can retrieve any souls if you lose. You can enrage the boss each time with magic or arrows.
This boss is required to progress in the long run, but it can be put off. The amount of trouble people have with it depends on the person, so I advise only trying it if you don't have many souls to lose. If you want to simply loot this area you'll receive the Gold-Hemmed set, which is extremely effective against poison and fire. If you want to avoid truly fighting the Ceaseless Discharge, you can simply run from where you pick up the Gold-Hemmed armour (at the end of the area), back to the fog wall at the entrance. This will cause him to fall, and you'll get the Victory Achieved notification.
Finally, the area accessible near the Hydra is connected to the Darkroot Basin and is also ultimately required to progress. This area, like the Demon Ruins can be put off until later. If you have 20,000 souls to spare however, I recommend buying the Crest of Artorias from Andre the blacksmith. This will unlock another entrance to the extended Darkroot Garden. If you do this you can farm about 7000 souls in a very short amount of time, and do this as many times as you like. You'll also be able to pick up the Stone armor and the Eastern armor here.
Ultimately, I would recommend heading to Sen's Fortress next as it is the logical progression of the game. If you consider it from a story standpoint, you wouldn't have reason to head to the Demon Ruins or the Darkroot Garden yet anyways.
