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)

/trustlist
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.
/trapped
if you are stuck inside someone's claim, this command will teleport you out after 10 seconds.
/unclaim
unclaims the current claim that you are standing in.
For a video on how to claim land, click here.