I thought that by using the White Sign Soapstone I will summon someone to help me beat a boss, I did beat a boss like that but instead of he helping me, it was like I was helping him. I appear as a Ghost and after the kill, the boss wasn't really dead for me. So, how can someone really help me kill a boss that counts to me?
The white soapstone you use, is to leave a mark for others to summon you to their worlds.
For you to summon someone to help you, you must be human and then you'll be able to see other's soapstone marks upon the floor, and activating them with square will then summon them into your world.
Be aware - when you are human, you can also be invaded by evil spirits who'll appear as red and will attack you (and be ignored by enemies).