Killing the undead dragon on the Valley of Drakes/Blighttown crossing on low levels is easy if:
You bring a LOT of firebombs
You bought some Fire Pine Resin from the undead lady AND you have a spear with decent reach (Winged Spear on Cementery/Pike bought from Andrei, normal spear is shit range)
You have pyBROmancy flame on a decent level
The undead dragon is very susceptible to flame attacks. let him vomit and then quickly poke him in its hand (Lock on not recommended). It's cool if you touch the toxic vomit as long as it doesn't build up, but it doesn't hurt to have Blooming Moss around (Bushmen drop em).
If you kill him and venture a little into Blighttown you can find a key to New Londo on a chest on the scaffolding, once you open that steel door on Valley of Drakes that it connects to New Londo and then Firelink.
I wouldn't brother braving the Drakes yet until you have ascended weapons or something that makes thing BLEED (Bleed takes away 33% of HP once it builds up).
You also need FAITH to with Astora's Sword, keep that in mind if you want to invest a little on it, specially if you are a Wanderer (which i am).
THIS REALLY SAVES YOU A PAIN IN THE ASS IF YOU HATED VALLEY OF DEFILEMENT ON DEMON'S SOULS, SINCE YOU'LL BE SKIPPING 3/4 OF THIS AREA.
But you'll be also skipping Ninja Set and Iaito (Badass draw stance katana), among other goodies.