You have to ring the second bell of awakening to get to sens fortress. Sens is the area attached to where Andre is. It has a large gate that will be opened when the 2nd bell is rung.
If you have the master key (best one to choose) you can get to blight town by taking the elevator down to new londo. After you reach the bottom to your right will be a tower with stairs. At the top of those stairs is a metal gate that can be opened with the master key. This will lead you to Valley of Drakes. Cross the wooden planks, fight 3 fattys, and your in blight town.
If you don't have the master key you have to go through the depths.
To get to Anor Londo you have to ring the 2nd bell, make it through Sens Fortress and beat the Iron Golem