To claim land, you can use a golden shovel. Do
/kit claim
to get a golden shovel (the claiming tool). Using it, you can set the corners of a claim.
Right click on a block to set that as the first corner to a new claim, and right click somewhere else to set it as a second corner, and claim the land
Any claim must be at least 5 blocks wide, and have an area of at least 100 blocks, or else it is too small and you won't be able to claim.
/trust [player]
adds a player to the list of trusted players in the claim that you are standing in. Use this to let your friends build in your claim.
( /containertrust, /accesstrust, and /permissiontrust as well - try those commands without a player name to see details)

gives you a list of all trusted players for the claim that you are standing in.
/untrust [player]
removes that player from the trust list of the claim that you're standing in- they will no longer have access to that claim.
/giveclaim [player]
transfers ownership of a claim to another player. This means they now own the claim and have full access.
if you are stuck inside someone's claim, this command will teleport you out after 10 seconds.
unclaims the current claim that you are standing in.
For a video on how to claim land, click here.