The AP Computer Science A course is taught in Java. In our course, we recommend using Eclipse as the development environment.
If you do not already have it, download the latest Java Development Kit here: http://www.oracle.com/technetwork/java/javase/downloads/index.html?ssSourceSiteId=otnjp
If you do not already have it installed, download Eclipse at: http://www.eclipse.org/downloads/ (available for Mac, Windows, Linux)
For more information about configuring and getting started with using Eclipse, visit: http://www.skylit.com/javamethods/faqs/Eclipse.pdf. This may also be a good resource to share with students as an early assignment to familiarize themselves with the environment.
Only a subset of the features of Java will actually appear on the AP Exam. Full documentation on the AP Java Subset is located here: https://secure-media.collegeboard.org/digitalServices/pdf/ap/ap-computer-science-a-java-subset.pdf. It's important to familiarize yourself with the subset before assigning any project or activity to ensure that students at minimum have multiple opportunities to practice using these features. You have complete flexibility in how you teach students to implement anything that is not part of the subset.