The Book of Ecclesiastes doesn't seem to have much of a pattern to it. But there are a few things that hold it together. The first half of the book describes the Preacher's investigations into wisdom (and folly and madness) and how he found that most things boiled down to "vanity"—they were empty, without purpose or lasting reality. The second half of the book shows his conclusions after he finished his investigation. This is only broadly true—some of the things in the first six chapters seem more like conclusions than parts of the wisdom investigation. But overall, that seems to be the outline.
Ecclesiastes begins by claiming to be the son of King David—which might mean he's only claiming to be from King David's line. This led to the widespread belief that he was actually the same as the famously wise King Solomon—a belief that Ecclesiastes, who certainly wasn't King Solomon, may have wanted to encourage so that people would treat the things he says more seriously. (Source)
After this brief opening, Ecclesiastes introduces us to his favorite word: "Vanity." Get ready. You're about to hear him repeat it… oh, a trillion times. He discovers that almost everything is "vanity": wealth is vanity, poverty is vanity, youth is vanity, old age is vanity. "All is vanity!" Another favorite catch-phrase is "vexation of spirit," which also translates as "chasing the wind" or trying to "herd the wind." He can't say that he recommends it. Pretty much anything you can think of amounts to trying to "herd the wind," too.
Part of the reason everything is so full of vanity, is because life is just a long repetitive process. Ecclesiastes talks a lot about how everything repeats itself, and nothing ever occurs that hasn't already happened before. Like the seasons, it all moves in cycles, and there's a time for every activity you could think of (except for cow-tipping, apparently—no one really does that). But the fact that everything is vain and empty and repetitive isn't supposed to make you sad—being depressed about the fleeting nature of life is just another vain waste of time and breath.
Ecclesiastes keeps saying to take delight in eating and drinking, to be merry and find work that you can do comfortably and happily. If you fail to seize the day like this, all you have are illusions—pursuing money for its own sake, or pointlessly struggling and competing to do better than the next person, to give a few of the examples he uses. You could wind up trapped in a glass case of emotion. Ecclesiastes says that you need to accept that God is far beyond the world where we live and where we'll never have a particularly great idea about what's going on. Ecclesiastes says to fear God and take him seriously, but doesn't really treat God in too much detail. He seems a little intimidated by the subject.
These are the biggest cars on Ecclesiastes's train of thought, so to speak. They run throughout the whole book. Occasionally, he runs through grumpy little digressions about how he thinks there are no honest women, or about how wise men just end up dying, the same as fools. Finally, he ends with a meditation on old age and on how "things fall apart," and death is inevitable. Cheery, eh?
But the big take away he intends for you to have is that life, when you try to understand it intellectually, never is going to make perfect sense. Yet, this frees you to take advantage of the moment and appreciate the simple things. If you can find someone nice enough to live with, and can get some meaningful work to do, things will be okay. ("It gets better," Ecclesiastes is saying.) You're probably going to be alright… unless, he says, shrugging his shoulders, it turns out you aren't. (He can never bring himself to say one thing for sure.)