what is the simplest way to write an app for IPhone?
At this time, there is simply no equivalent of B4A for iPhone. Xojo should have an iOS version later this year, but no date is announced.
Apple provides a set of free tools for Mac OS X at
http://developer.apple.com : Xcode is the language used to develop in Objective C, and it contains Interface builder, which lets you design the interface. But Objective C is very far from Basic and considered difficult to learn by many.
The easiest solution I found is Livecode at
http://livecode.com.
It is a language close to English, more so than Basic, probably just as easy to apply. The IDE is available as community edition for download (registration required) for Mac OS X, Windows and Linux.