Unless you are volunteering, your school will typically provide you with the basic necessities you need to teach a class: Text books/reference materials, curriculum, white board/chalk board/markers/erasers, flash cards, worksheets, story books, ESL games, etc.
More advanced schools will have CALL Labs (Computer Assisted Language Learning), Internet resources, online collaboration, and other technological advances. Most schools will fall somewhere in between. It is best to prepare by asking other current ESL teachers within the organization as to what type of resources are available and what is lacking.
Visit the Prepare for the Classroom section of the Preparing to Go Checklist to ensure you are all set for your first week of teaching.
For more ideas on resources for the classroom, visit Oxford Seminars ESL Teaching Resources.