You progress up the forest into the castle, the second bonfire there (with the old lady), go down ladder, and progress around the castle, you will come out on the other side of the ladder (there is locked metal door right next to the ladder, that opens up when you go around it, so you get a shortcut to the boss). There will be an elevator, step on it, and it will take you down to the first boss, the giant.
Remember to drop down the ledge after you kill the pursuer (you kill him after the giant, as the giant gives you a key to open some of the locked doors in the castle) to get great armor, and the first 100% damage reduction shield, it helps tons. Remember to boost the stat that increases weight capacity.
After that, you can go to the Heide Tower, and kill the dragon rider. There are several large mobs there, they hit hard, but their attacks are easy to follow. 3 of them activate up triggers that you can push when you kill them. Two of them extend the platform for the Dragonrider fight. The third one (when you go to the left) is at the top just before the cathedral. When you kill him, the button that comes up lets you activate a drawbridge that gives you the ring that limits the amount of hp you can lose as undead to 80%.
After you kill the dragon rider, you can go to the wharf (enemies there are around the power of the first forest/castle).
You can also buy the ring from the cat (lowers falling damage) and jump down the well in the city. Need the ring to survive. Recommend lighting a torch before you do it, so you can see where to go.