Here are some platforms where you can start your wiki for free. They are among the best available for teachers :
1- Wikispaces :
This is a free wiki host providing community wiki spaces, visual page editing, and discussion areas. It is my favourite platform and it is the first one I would recommend you try for your class.
This is another great wiki hosting platform that lets anyone sign up and create a new wiki but the free version is ad-supported.
Examples of Educational Wikis
Wiki video tutorials:
1- How to Build an Educational Wiki
2- Using Wikis in The Classroom
Exemplary Classroom Wikispaces
Tips for Teaching Wikis
The Way of The Wiki
Cybrary Man Wiki Resources
Wikis Connecting Education
10 Best Practices for Using Wikis in Education
17 Interesting Ways to Use a Wiki in The Classroom
50 Ways to Use Wikis for a More Collaborative and Interactive Classroom