Whenever you want to. Talk to an estate agent - there are estate agents in Varrock, Ardougne, Falador, and Seer's Village - and pay a small amount to get your house. You must be a member.
no you can not sell your house on runescape.
No, Your RuneScape House is based on your Construction Level.
Camelot
Go to the place with the house icon in varrock.
in the volcano at michaels house
Yes.
open the door
Yes.
you build on your house
yes
No.
Speak to estate agents in Runescape. One of them is in east varrock just opposite the altar