The first lecture of a course is perhaps the most important lecture. It is the lecture that will set the tone for the term and will affect many things. It will modify and set expectations. It will give the students some inkling as to your style, ability to communicate, and it will also inform you about the class and the students.
Students do not come into the class memoryless. They will have expectations and pre-existing perceptions about the course, the course methodology, and course delivery. The expectations will be based on upper year feedback, what might be known about course evaluations, and what courses have been taken. Whenever there is a change in instructor or in expected outcomes (e.g., fixing a course widely perceived as a bird course), the incoming expectations will be important to deal with. A core course will have a set of expectations different from an elective in the program and the expectations will be different for electives taken in other, non-program areas. The expectations will also vary based on year. Junior students will have different expectations than senior students. Lots of expectations to be aware of and to deal with. The students will have perceptions about you, value of attending classes, value of the text book, value of assignments, difficulty of assessments, and so forth.
You, if possible, should have a handle on the incoming expectations and perceptions and either confirm them with the students or deal with them. You might be a new instructor for the course, with a different text book, new assessments, new desired outcomes, new methods, and so forth and these should be identified and discussed in the first lecture. There should be transparency. What is different, what is myth, what is reality, and why are things different (if they indeed are). Of course, some students will not be listening, engaged, or willing to believe; they may be in denial. The worst situation is where the students have elected the course (not a requisite) and they have elected the course based on their expectations, only to find that the game has changed. If the students have no option, they will not be happy campers; if the effort is higher, content more difficult, marks not as easy to get.
Unfortunately, any mistakes made in the first lecture are hard to recover from. The initial impression will be dominant and it does not really matter what the instructor says or does subsequently to fix the situation, some, perhaps most of the students will have one memory and one memory only.
The first lecture also has to set the course and plan for the term. It establishes the value-mapping, motivation, and rationale for why the students should engage and ‘learn’. They should know the learning outcomes and the quality expectations of the instructor. The students should know the assessment schema and planned teaching methods. They should know when things are due and the rough expectations per assessment (e.g., average student will need to spend nn hours to get an average mark). The students should know what is assumed to have been learned in earlier courses; e.g., we will be using blah blah that you learned in blah blah year in blah blah course.
They should know how the TAs will work, how you will work with the TAs, etc.
Given what should be discussed, there is little room for actual ‘new’ material in the first lecture. If necessary and appropriate a short assessment that informs you about the students’ intellectual abilities and informs the students about your style/methods is also a good thing for the first lecture.
The introductory lecture will be somewhat the same but different depending on the year, students enrolled, and past history.
You should consciously spend some time designing the first lecture to deal with tone setting, expectations, and creating the relationship with the class. It should not be left to nature.