There’s a lot of history from Tolkien’s additional works like “The Silmarillion”, so I’ll try to make this as simple as possible. Melkor is a powerful member of Ainur, also known as the Holy Ones. They are the first immortal beings created by the supreme being of the world, Eru.
Long before Elves, Dwarves or humans walked the land, Melkor wanted to rule. He resented his brother being given the replacement post. He rebelled and became known as the Dark Lord of the First Age through his many evil deeds.
Melkor tries to destroy the world, stealing the Silmarils; gems made from the essence of the Two Trees of Valinor. That theft killed the trees, which were the source of light to the world before sunrise. (It’s mentioned in one of the trailers, with pictures of those trees you can see above.) Their final flowers created the sun and moon. For his actions, Melkor received the name of Morgoth.
Upon discovering the newly created Elves, he captured some of them and tortured them to create the first Orcs. He created Balrogs (which you’ll remember from the “LOTR” trilogy), and possibly (depending on which story you read) also creates dragons.
All of this, combined with his other evil deeds, led to a great war. Morgoth was eventually captured and banished from the world into an endless Void. However, the apprentice Sauron remained free, despite being concealed.
https://www.slashfilm.com/982502/who-is-morgoth-the-lord-of-the-rings-villain-explained/ The Lord of the Rings Villain Explained