Once you are level 14, have gone to your main faction city and acquired your housing plot, you can be transported to your new land in the sky. Here you will find construction debris and a handful of various trees. Congratulations.
Provided you have spend your funds cautiously, you will most likely want to exchange the mess in the front center of your land for a small house. Looking at the lower left corner of your UI, you should see a new window with options, one being landscape. By bringing this up you will see 7 "sockets" of various sizes. One being a larger square, then four smaller squares and two large rectangles. These sockets accept FABkits appropriate for the size of socket they are fitting into. Housing Challenges only fit into large sockets, the BBQ pit and gardens only fit into small ones. If you choose to build a new house, it goes into the larger square socket in the front and center of your land, replacing the construction debris. Clicking on a socket in the landscape window will bring up a list of FABkits in your inventory (previously acquired) and any pre-made kits available for immediate purchase. The list is long, so checking the circle that says "only what I can build' will immediately reduce the list to a usable size. Once you confirm your chose and the money (if applicable) has been exchange, building begins. This takes only a few moments and soon your new building/challenge/whatever is ready for your enjoyment. Some "plugs" like your house are permanent and do not require upkeep, but many of your other assets may not last longer than a week before needing repair. If left unrepaired (yes there is a fee for this, how else is Protostar gonna feed the kiddies?) the plot could become unusable. Sockets holding only visual items or decor are the exception as they do not actually do anything or provide a service of some sort (i.e.: gardens or tradeskill related plugs). What this means is that a crafting table (BBQ pit) or ore fields provide harvest-able items but will no longer replenish if not maintained.
Once a plug or FABkit is installed, it can be replaced with others of the same size, but will completely destroy anything currently in that socket. You will be asked to confirm the destruction before the building begins. Once this is done, there is no going back and any previous kits will need to be repurchased or re-gained to put them back in.
Decor can be added inside and outside the house (some are limited to inside only) and is limited only by the size of the socket. The outside edges of the plot are currently unchangeable and will not accept any decor items.