Who was Agamemnon?
This is a difficult question to answer, because Agamemnon was a figure that lived so long ago that there are only legends about him. In view of this, we do not even know if he really existed. He may have, but we cannot be dogmatic.
What we can say about Agamemnon is mostly from Homer's Iliad, and a few other works in Greek literature, such as the tragedy that bears his name.
The rough outline of his life is as follows. He was considered the greatest king of the Greeks, the brother of Menalaus. He led an army to fight the Trojan to get back his brother's wife, Helen. He eventually defeated the Trojans with the help of Achilles, whom he always butted heads with.
In the end, he is said to have been killed by his wife, Clytemnestra, because he had to sacrifice his daughter to get favorable winds to sail to Troy.
According to some websites, Agamemnon was the son of King Atreus and Queen Aerope Mycenae. Mythical legends state that he is the King of Mycenae or Argos. During his return from Troy, he was murdered by Aegisthus, the lover of his wife, Clytemnestra. This is based on the fullest version of the oldest surviving account, Odyssey Book 11, l.409f.
This information is from one of the websites listed below. The websites will provide you with detailed inormation about his life.