The first section, as the section title indicates (The Burial of the Dead), is about death. The narrator is surrounded by a desolate land full of "stony rubbish."
The next section, "A Game of Chess," transports the reader abruptly from the streets of London to a gilded drawing room, in which sits a rich, jewelry-laden lady who complains about her nerves and wonders what to do. the poem then switches to a pub where two Cockney women are talking. In just a few words we move from the upper crust of society to the dregs.
"The Fire Sermon" opens with an image of a river. The narrator sits on the banks and muses on the deplorable state of the world.
"Death by Water," the fourth section of the poem, describes a dead Phoenician lying in the water -- perhaps the same drowned sailor of whom Madame Sosostris (in section one) spoke.
The final section of the poem, "What the Thunder Said" calls for rain...a cleansing of sorts...and it comes.
The general theme is desolation, decay, and ruin of society through war, vice, and the dark side of human nature.
There simply is not enough space here to explain "The Waste Land" adequately. Please read the eNotes study guide has excellent articles on the themes, style, and historical context of the poem, as well as critical essays on its structure and literary impact and a critical overview.
As for writing your essay, you might find the article "How to Write a Good Essay on English Literature" helpful. I've included a link to that page in the sources section below. I've also pasted a link to the Oxford Companion to English Literature article on the poem.
There's no one answer that can fully answer what Eliot's poem is about. However, a key to understanding _The Waste Land_ is to recognize that after World War I nothing was the same. The world was a fragmented place and Eliot was trying to make sense of the world. He saw the world as broken and cruel, mocking humanity because it could regenerate but humanity (once born) cannot.
Despite this, the poem does end in cathartic fashion with the Indian word for peace. It's a beautiful, challenging work of art!