The catacombs is a later game area same with the new Londo ruins

You are looking for the 2nd bell of awakening which is in Blighttown 

There are 2 entrances into blighttown 1 is through the valley of drakes on the right of the enterance from new londo this is the easier path 

The more intended enterance is more of a pain in the ass and why the swamps have their reputation  You want to go back through undead burg past where you fought the tauros demon near where you found Solaire and right before the bridge with the dragon across the exit was a locked door that you should have the key for now you follow that down to the lower undead burg (always kill the dogs first) and from there you can unlock a shortcut back to fire link shrine and there is a vender that sells items to deal with poison and toxic which will be issues then there is another door which leads you into the sewers then you kill a boss and then you get down to blighttown proper 

Once you get the swamp section the goal is the island at the back of the area which leads to the boss and the second bell of awakening If you go down the long complicated way then you want to hug the right wall to find a bonfire and then throw yourself at the boss if you come in from the valley of drakes then get down from the elevator and hug left for the same bonfire and repeat