Claiming

How much land you can claim is limited by the number of 'claim blocks' you have. You'll automatically get claim blocks by just playing the game. Holding a golden hoe will show you how many claim blocks you have left.

You can check who has claimed an area of land by right-clicking in it with a feather.

Here's how to claim some land:

Acquire a golden hoe with /kit claim - this is the tool used to claim land.
Go to the area you wish to claim. You can only claim rectangles of land.
Right click with the golden hoe at one corner of the rectangle you wish to claim.
Right click at the other end corner of the rectangle.
Profit.