Do you mean how do you write a program that will run on top of Linux (or RiscOS or another operating system) on the Raspberry Pi or do you mean how to you write an operating system or code that will run on the Raspberry Pi without another operating system underneath? If my question is confusing or you don\'t know the answer, then you want the first one.
Raspberry Pi can run several different distributions (basically \"versions\") of Linux so there will be lots of ways you can program it. If you already know a programming language, chances are that there\'ll be a version of that language or something similar for one or more of the Linux distributions. If you don\'t already know a programming language, then there will likely be some easy-to-learn programming languages included or easily accessible. For example, of the two languages I originally learnt, Pascal was my favourite. Free Pascal looks to run on Linux on ARM processors (which is what the Raspberry Pi uses), so because that\'s a language I know then I\'d download that and start programming. If you know C++, the tools will probably already be built in.
Are those the sort of answers you\'re looking for, or are you looking for something more basic/beginner?