To start exorcism you need to learn it from Pedro. You will find him on Isola Tiberina (go south from Piazzia Venezia where Tamis used to be)

You will need the following stuff:
1. Cross
2. Rosary
3. Holy water
4. Exorcism book

Each level of exorcism skill allows you to exorcise people with possession 10 multiplied by your skill

IE.
If someone is possessed in 20% you need at least skill level 2.
If someone is possessed in 85% you need at least skill level 9.

To perform exorcism just go next to the player and click on it like you want to chat than click Exorcise button in the top-right corner of the screen (in Android version)

Notice: Success or failed exorcism requires 1 holy water from your inventory so make sure you got the proper quantity during you quests.