You'll want to find out if students have access to computers outside of class before requiring that work is completed on online.
Decide how you'll evaluate writing on discussion forums (as class participation, as homework ...). If you don't provide incentive for using the forum, fewer students will write substantial comments
Know that some students may lose sight of the "academic context" and offer inappropriate comments on the forum (For example, a student may use the class forum as a way to express criticisms of the course when he/she should address such concerns with you in a less public context). Be sure to establish appropriate guidelines for using discussion forums. Also, inform students of any expectations you have