In a well designed course you want the students to hit the ground running. This has not always been typical of online courses in the past, so make them aware this is not going to be an old fashioned click, watch, submit type of course. Let them know right away that you have expectations which include involvement and abiding by deadlines. Not addressing this right away will lead you to ultimately having a few students who think because it is online, they can jam pack 15 weeks into one at the last minute. This has no benefit for them or their classmates, and will not even be possible to attempt if the course is designed with interaction in mind.
In order to shoot across the bow (so to speak) to wake your students up, contact them two or three weeks before the semester or course begins. By being proactive you will set the tone that you are going to expect them to be involved and active in the course. The contact could be something as simple as a welcome message/video (if you can, make it personal, i.e. "Welcome to 101 Dave") via email. For some, that may have registered a month or two before, it will also serve as a reminder that they signed up for the course.
A welcome email will also serve to make your online students feel invited and help jumpstart the online community you want built. Picture two scenes in your mind, June Cleaver (google her if your too young to remember) welcoming you into a house with fresh baked cookies and a warm smile, or walking into a house where the door mysteriously creaks open, it's dark, musty, and you are left asking an echoing call, "is anyone home?". Which of those two scenes would you prefer to enter in...which one will represent the start of your online course?
Until then have a great weekend and don't forget, the best part of waking up is...waking up, "This is the day which the Lord has made; Let us rejoice and be glad in it." ~ Psalm 118:24