Overview
As explained in The Silmarillion, the First Age began with the Awakening of the Elves, and it ended with the final overthrow of Morgoth by the combined armies of Valinor and Beleriand. It covered a long period of Valian Years, followed by approximately 590 Years of the Sun. Depending on the choice of conversion factors (among many that Tolkien used at different times), this translates to a period anywhere from 4,902 to 65,390 solar years. The greater number is supported by the Appendices to The Lord of the Rings and later writings, the lesser by earlier writings. The First Age was also known as the Elder Days, although this term was (starting with the Fourth Age) also applied to the first three Ages combined.
A common misconception is that the First Age started only with the Years of the Sun, and it is sometimes referred to as the "First Age of the Sun", beginning with the first rising of the Sun and continuing until Morgoth's defeat. This interpretation is not supported by any of Tolkien's writings, and contradicts references to the First Age being the longest age by far.
Tolkien describes primarily the events that occurred in Beleriand, or in his words, "the last six centuries of the First Age." These centred around a series of wars waged by the Sindar, the Noldor and the Three Houses of the Edain against the armies of Angband and the evil Men from the East. The wars had actually begun during the Years of the Trees, but continued after the arrival of the Noldor in Beleriand. There had been Elves in Beleriand for uncounted millennia, and they warred with Morgoth after his return; but the Noldor, particularly the Sons of Fëanor, had come with the express purpose of defeating Morgoth.
Read more about this topic: First Age