Claiming your land is an important step to protecting your buildings and your items. Its also very easy to do!
First you need a golden shovel, you should have spawned with one but if you lost it you can get another one with /kit tools
While holding the golden shovel, right click at each of the corners to mark out your claim. You can view who has claimed a block by right clicking with a stick and it will show an outline of the claim.
Claims extend from sky to bedrock so you dont need to worry about players building above or below you! You have a limited number of claim blocks but you gain more slowly while playing on the server. You can also buy more with /BuyClaimBlocks if you want to skip the wait.