Is salt necessary in our diet? What for? What is the necessary ammount?

yes, 1500mg/day is recammended by the national academy of science. It helps to regulate the exchange of water between cells.

See below for further reading:
http://www.saltinstitute.org/27.html
http://seagrant.gso.uri.edu/factsheets/salt.html

But be wise, to much salt is not good for you. Does anybody know how to balance this out. Like when you have too much salt what do you take?

YOur body does it automatically through sweating. The kidneys are also involved in the filtation process; If you have alot of salt, the kidneys filter is out and it comes out in your urine, if you don't have much salt, the kidneys retain the salt.