This wiki article will help you understand how to protect your area.
Claiming is done on a chunk-basis, one chunk in Minecraft is 16x16 blocks. Your claim always extends from the bottom of the map to the limit of the sky. More information about chunks can be found on the Minecraft wiki here: Chunk
There's a lot you can do with land claiming, but let's start with the basics here.
Assuming you're a new player and have never claimed land before, this is what you do to protect your area:
/claim
/view
to show the borders, walk outside and type /claim
againThat's the simple basics to get you started! Want to manage and change settings for your new area? Just type /lands
to open the GUI menu.