What: Setting clear expectations and a rationale through the syllabus and on the first day of class. This can include the course goals, recommended pre-requisites, assignments, readings, deadlines, class attendance, participation and any other expectations.
How: In your syllabus you might want to think of ways to positively frame expectations for the students and also state your promises to the students so they can have a sense of your commitments to the classroom. You might also want to explicitly share your commitment to working with students should they have any accommodations or feel like they might need extensions or modifications.
Why: This exercise can be helpful for you to articulate your own goals for the course. And it will allow students to know the amount of work expected for the course, if it meets their own goals for taking the class, and if they are prepared to take the class.