Inventory all of your books. Record the title, including the edition number, the authors and the ISBN number. Creating an Excel spreadsheet to record your books would be the simplest way, but you could also just write the information on paper.
Research, using your favorite search engine, "sell college textbooks". You will get many choices. As you research, look carefully to determine how shipping works. You want a site that will pay for your shipping, as shipping books can be expensive, and you will end up not making a profit.
Research the company that you choose by checking online. You don't want a company that has a lot of bad press online.
Box up your books and send them to the company you selected. Your check for your books will be mailed to you.