How do I level my plaster walls and to what level?
It would be a good idea to install a tile backer board over the plaster. Over time, plaster dries out and becomes unstable. This could eventually cause your tiles to loosen or to pop off.

I would also recommend putting a thin layer of thinset mortar on the backside of the backer board. This will help attach it to the wall and the mortar will fill in any inconsistencies in the surface of the wall. Once the backer board is installed, you will have a great surface to attach your tile to. The screws in the backer board will also firmly attach it to the structure of your house, and it should basically last forever.