Start off your classroom with an icebreaking activity -- preferably one that is a bit silly but allows your students to get to know each other as a group -- it goes a long way toward creating a safe, fun environment in the early stages of your class time together. Icebreaking activities can be as simple as having two students interview each other and then introduce one another to the larger class, or more complicated, such as having each student tell the class two truths and one real-sounding lie and then letting the rest of the class quiz the student with yes and no questions.
Outline your participation expectations on the first day of class; if you grade based on participation, make that clear in your syllabus and make sure your class understands. Ensure your class understands what your definition of "participation" means. For some teachers, this means being involved in classroom discussion; for others, this means a variety of in-class and online activities. Make your expectations clear early on.
Laugh with your students. Learning should be fun, no matter what the subject. If you have fun and are passionate about what you are teaching, your students will sense that. If you can laugh at yourself, your students will be much more at ease. Have a respectful sense of humor and keep your classroom as fun as possible, while maintaining a sense of control over your class.
Teach with a variety of learning styles in mind; no student learns in the same way, and what piques one student's interest and causes her to interact may not work with another. Incorporate a variety of learning styles into each lesson to encourage each and every student to participate. A short lecture will capture the attention of those who learn through hearing; a graphic presentation -- even a simple one, such as drawing the concepts on the whiteboard -- will reinforce the ideas with your visual learners. Really drive your lesson home by having all of your students practice the concepts in an experiential learning segment. By using a variety of styles, you will encourage students to actively participate.
Break students up into small groups to complete activities; this helps them get comfortable talking with their peers. Once they've discussed concepts with two or three others, they will be more receptive to sharing ideas with the larger class group than they would had they not prediscussed it with a smaller peer group.