Legolas is the son of the king of Mirkwood, Thranduil. Nothing is known of his mother, other then she perished during the War of Wrath. It is also uknown if Legolas had any siblings, thus it is unknown if he is the heir to the throne of Mirkwood or not.
Elrond, Lord of Rivendell called together a council when a Hobbit by the name of Bilbo Baggins brought the One Ring to Rivendell. At this council were Elves, Men and Dwarves. Elrong explained that the Ring must be destroyed, and the only way to do that was to take it to Mount Doom and throw it in the fires where it was made.
As the journey was too perilous for just one to handle, Elrond concided that all join together to help each other. There was a total of nine members including. There were four Hobbits: Frodo, Sam, Merry, and Pippin, two men: Aragorn, and Boromir, one dwarf: Gimli, one wizard: Gandalf, and Legolas was the lone elf. The nine were brought together to destroy the One Ring.
The Battle of the Hornburg, or more commonly referred to as, The Battle of Helm's Deep was fought at Helm's Deep. Saruman sent his army of 10,000 Uruk-hai to extenguish the people at Helm's Deep. Theoden, King of the Rohirrim, brought his 1,000 soldiers there to defend his people. An army of 500 Elves from Lórien arrived to help the Rohirrim just before the Uruk-hai were there.
Legolas, Gimli, and Aragorn had since been seperated from the other four Fellowship members (Boromir was killed). They found themselves in the open lands of Rohan. They helped the people of Edoras make their way to the much more fortified Helm's Deep. Once there, they decided to stay and help protect it. As the battle drew on, all seemed lost, that is until Gandalf arrived with the entire Rohan army of 2,000. The Uruk-hai were forced to retreat and Helm's Deep was saved.
The Battle of the Pelennor Fields was a battle for the city of Minas Tirith, and ultimately for Gondor, during the War of the Ring. It was the greatest battle of the War of the Ring, and indeed the largest of the entire Third Age. Major casualties resulted from this battle - including the deaths of King Théoden and the Witch-king of Angmar.
During the initial skirmishes, Legolas, accompanied by Aragorn and Gimli, went to the Path of The Dead. In there, was an undead army waiting to be set free. Only the heir to Gondor's throne could do so, which Aragorn was. He told the dead if they fought for him, he would grant them their freedom. Upon arrival, Minas Tirith had already been breached, but not outrighted defeated. The undead, along with Legolas, Gimli, and Aragorn were able to save the city and drive the reaming Sauron forces back to Mordor.