reem, you probably don't have to worry about Fornost in regards to reading Lord Of The Rings.
I believe that when you read
The Silmarillion, you will understand it better.
I will tell you it anyway:
In the year III 1974, the kingdom of Arthedain was destroyed, and with it the last
realm of the Northern Dúnedain. The
armies of
Angmar and its allies overran the northern lands, capturing the capital of the Dúnedain at Fornost, and King Arvedui fled into the icy north. It seemed at that time that the
Witch-king's victory was complete.
In the following year, though, the tide turned, though too late to save the remnant of the
North-kingdom. The
ships of a great fleet sailed into the
harbours at
Mithlond, carrying a force of Gondorian warriors sent north under the command of the great general Eärnur. To that army joined the
Elves of
Lindon and the remnant of the Dúnedain, and they marched east to avenge the loss of Arthedain.
They met the Witch-king's armies not at Fornost itself, but on the
plains westward towards Lake Evendim. Aided by Eärnur's military skill, and a force sent out of
Rivendell led by
Glorfindel, the armies of the Dúnedain and Elves had utter victory, and Angmar's armies were swept away. The Witch-king himself fled into the dark of the falling night, and Glorfindel famously prophesied that he would not fall by the hand of man, as later proved true in the
War of the Ring.
The
Battle of Fornost shaped the future history of
Middle-earth in many important ways. Its most significant effect could hardly have been guessed at the time: it robbed
Gondor of its
Kings. When the Witch-king fled south, and came at last to
Mordor, he did not forget his defeat at the hands of Eärnur. Many years later, Eärnur became the
King of Gondor, and the Witch-king challenged him to single combat. He refused, but seven years later the challenge was made again, and this time Eärnur accepted. Riding to
Minas Morgul, he was taken by the
Nazgûl and never heard of again. He was the last King to rule the
South-kingdom until after the War of the Ring nearly a thousand years later.