Many writers develop ideas from reading. For example, what do you think about a recent magazine article about manufacturers moving out of the U.S. to keep costs down (to pay workers $1.00 a day instead of $18.00 per hour)? What do you think about a newspaper editorial that is for/against quotas to ensure equal employment? What do you think about the idea, offered in a college textbook, that the U.S. is a society dominated by a traditional class structure? Reading can spark lots of ideas for writing, and it's a sure bet that you will be asked to respond to certain assigned college readings with your own ideas.
You can prewrite for an essay by writing your ideas down as you read. Record your thoughts in the margins. Agree or disagree with others' ideas, and jot down your reasons. Jot down questions that occur to you as you read. In essence, carry on your own dialogue with the writer of the text, as though you were talking with him or her, and write that dialogue down so you can retrieve it later on.