Reading.
When "first-year writers" are asked to read something, it's often in service of providing subject matter to write about. In a book I used to use in my courses, my students were told to craft their own writing in response to something somebody else said. This book, called They Say / I Say, had a number of virtues On one hand, yes. It's important to be engaged in conversation with others. Dialogue is useful for all the reasons that Socrates noted in his famous philosophical works. He may indeed have supplied us with the single best method for thinking critically, but such dialogue is most useful (and most substantive) when the parties are sufficiently informed or have something to bring beyond an initial opinion--which is to say, when both parties can contribute to the knowledge they produce through dialogue with knowledge of their own.
Here I want to distinguish between experience and knowledge. We all have experiences that inform our thoughts and responses to what we see, hear, and read. But experience alone does not constitute knowledge. And you're in college to gain some. Much of it will come from reading and the experiences you gain concurrently. Neither one nor the other is complete on its own, but reading widely about something can help you gain knowledge about something you haven't experienced. Reading widely can also help you convert something you experienced yourself into knowledge, if only because what you read can broaden your own subjective sense of what you experienced with other perspectives.
Reading is necessary because we can't know everything just by what we've lived through or what we've read by a certain age. We can't even know that much in the grand scheme of all there could be worth knowing.
Reading is also necessary because it allows us to see how other people articulate knowledge, and, therein, it gives us a sense of how we, too, can formulate our own ideas.
To say so is not to say we all plagiarize from one another, but rather, to admit that we learn what constitutes good prose by reading it and imitating the style, structure, and diction we've seen elsewhere. I owe the strength of my own writing to reading for precisely this reason. I'm certainly not telling to you to copy the sentences of published writers and submit them as your own work. I am telling you, though, that the more you read, the more ideas you will have about how to express your knowledge in written form.