Depends what you want to build etc.
You can read about the two main supported APIs:
- desktop: http://docs.maidsafe.net/safe_app_nodejs/
- web app: http://docs.maidsafe.net/beaker-plugin-safe-app/
Mobile is not supported yet, but MaidSafe have started work on a Java based API (I forget the precise flavour).
There’s a developer forum which is great for questions about the APIs and has extra information for developers such as:
You can test your apps live with Test 19 or use a local development setup such as mock-routing (as in the above “How to”).
Hope that helps