Buying a house is very simple, you just have to check out the list of houses, select your world, city and available and then click on Submit.
Now a list with all empty houses of that city will appear (if there's any empty).
If a house seems interesting click on "view" to see more where exactly it's located, if you want to buy it, click on the button "purchase/login" and follow the steps.
If you fulfill the requirements to buy a house you will be notified you have bought the house.
You will be set as owner of the house directly, but you will get access to enter the house first next server save!