how do i change a timing belt on a 1994 Toyota corolla and what do i need to buy

This guide can show you how to do the job, scroll down

https://www.2carpros.com/diagrams/toyota/corolla/1994

Please let us know if you need anything else to get the problem fixed.

1.8 liter 4 cilinder

You should be getting a timing belt kit. this will include belt, tensioner and seales to do a full job.