As ThisJKid said, you have to be in a faction to claim land. You must either be a high rank in the faction or the owner of the faction to claim land.
The commands are /f claim (which claims a chunk of land) or /f auctoclaim (which claims all the land you walk on).
In order to claim land, you have to have power. You can check power by doing /f p. You gain power by the hour. I believe it's 4 power per hour. You lose 3 power upon death. The more power you have, the more land you can claim.
So if you have, say, 1 power, you can probably only claim a very small amount of land. But if you have 10 power (that's the max), you could do /f claim many times.
In order to unclaim land, you do /f unclaim, which unclaims a small amount, or /f unclaimall, which unclaims all of your land. This also means your /f home will be gone, so be careful about that.
Hope this helped!