Good places to help:
Bonfire behind locked door in sewer right before gaping dragon
- Why: Easy path to the boss and most people turn human there. No one invaders in the sewer, people are likely to want help with the boss to cut down on how long it takes. They might want help with the curse frogs to explore that area.
Bottom of blight town in the sewer pipe
- Why: Easy path to the boss, most people will turn human at the bonfire and immediately summon people. Unlikely to be invaded in the sewer. Larger level range due to higher level. Very likely to have humanity and want to summon people to finish off blighttown.
Before the large stairs in Anor Londo
- Why: Path to the boss is fairly difficult and the double boss fight is made much easier with help. Slightly less good because of the more common invasions. Larger level range and almost everyone passes through this area around the same level.
The last bonfire before the crystal cave in duke's archive
- Why: Difficult if they don't have the shortcut open and confusing. Difficult path to the boss and a helper will make it more likely to pass and you can show them where the invisible bridges are.
Either bonfire in tomb of the giants
- Why: As a sunbro you are likely to have a divine weapon and can permanently kill the skeletons during the boss fight. Also your lightning spears makes killing many of the skeletons and traps much easier.
There are some of the major ones that I have found success in.