Land claims automatically protect all the way to the sky, and a couple of blocks below the lowest block you've placed, so you're safe to build things like a underground base or basement. You are given a limited amount of claim blocks when you start, however you earn more by playing. You get 100 per hour of un-afk play time. You can check your total with /claimlist. You can also get more by donating or voting.
Step 2: Click opposites corners of you're house (or area) like in the photo below.
Step 3: If you've done it right, you'll see some gold + glowstone come up on each corner around the selected area, everything inside this area is fully protected from grief.
To check if your land was successfully claimed, use a stick and right click the nearby ground. This can also be carried though in checking other structures before raiding. This will help prevent any issues with raiding/griefing.