You must have been good enough since you passed their interviews and exams.
If you want to prepare I would find out the technology stack they are using and start learning it. Find online courses from Udemy, Pluralsight, code academy, etc. and start learning. Make sure you actually setup a development environment and do sample projects.
Make sure you understand modern source control systems like Git.
Buy the book Cracking the Coding Interview and learn all the basics of data structures.
Most everyone learns on the job. Make sure you listen, ask questions when you don't know something and have a good positive attitude.
You will do fine.
Source: Quora