Most colleges and universities in the US use Learning Management Systems (LMSes—these are systems like Moodle, Canvas, BlackBoard, etc.). When I teach a class, I upload any course materials intended for students to whatever LMS my institution uses. If your institution uses an LMS, I would suggest that your first stop should be there—log into the the LMS, and see if your instructor has posted anything.
After you have checked the LMS, then it is reasonable and appropriate to email the instructor if there is some material which you think might be missing. Send the email from an institutional email address—many universities and colleges have pretty aggressive spam filtering, and messages sent from outside of the institution might be "quarantined" or never delivered. Keep it short, don't make demands, leave out irrelevant details, and be specific:
Dr Henderson,
I am enrolled in [Course Number and Section]
. I have checked the LMS and see that you have posted [a syllabus | notes | whatever]
and that the text book for the class is [whatever]
. Are there any other materials that I need or that you can provide to me before the semester starts, such as lecture slides or homework assignments?
Thanks much,
[Student Name]
Note that the response is very likely to be "It is all on the LMS" or "It will be on the LMS shortly". Note also that some instructors will be happy to share additional materials with you, and others will not. Some instructors have slide-decks or written notes that are prepped and ready to go long before the semester begins; while other instructors (like myself) tend to leave preparation until not long before the class actually meets, in order make changes in response to class performance and needs.
I would also be slow to follow up. The beginning of the semester is often a rough time—classes are being moved around and reassigned to different instructors in order to meet student demand, instructors are prepping for the first day of class and are very likely overwhelmed with requests from students, administrators, advisors, and other parties. My own institution has a policy of requiring that emails be answered within two business days, so following up on the third day is not unreasonable, but it is also worth it to give your instructors grace and understanding—they are probably almost as stressed out as you are right now.