To claim land you need a golden shovel. (we plan on changing this to a wooden shovel soon)
If you havent made a claim yet one will be automaticly created when placing your first chest.
To create a claim you need to use the shovel and right click a corner of the area you want to claim and then do the same on the other corner and you have created a claim.
To modify a claim you need to right click a corner of a claim to resize it and then right click to the spot you want it at.
A video with instructions can be seen below.