One of the biggest, if not the biggest adjustment for students when they take online classes is the absence of their instructor and classmates.
All of your communicating will be done by typing on your keyboard, so you need to be especially aware of what you say (type) and how you say it, which includes discussion boards and email.
The majority of classes that are taught online are going to require your regular participation. If you are used to walking into class,sitting in the back, popping in your earbuds or texting your friends, you can do that, but you will still be responsible to participate during the week. You might be asked to post answers to questions in the discussion forums and you might even have to work in virtual groups.
Because so much of what you do will be written communication online, you should become familiar with rules of netiquette: The Core Rules of Netiquette
Your instructor will not be there to understand you are confused, lost, or frustrated. You need to communicate with your instructor. Check the syllabus for their contact information and do not feel bad for calling, emailing, or posting questions in a virtual office forum. They are there to teach you, and if you are lost, you aren't learning. Speak up!