How do I claim a place in the actual server building place thing

Let a staff member know the coords of the spot you're interested in. They can double check to make sure it's ok to build there.
You can mark the border of the land you want to claim with a glowstone border. Doesn't have to be a solid line. Also, make sure the border follows the natural curves of the landscape - no square or rectangle borders.
Once you've built a significant amount on your land, you can have a staff member look at it and if they approve, you can have your area regioned and protected.