The messages that you type in are in a computer language called BASIC (standing for Beginners' All-purpose Symbolic Instruction Code). It actually takes the computer quite a lot of effort to break down the BASIC messages into its own rudimentary operations, but, after all, that's what it's paid to do. The BASIC messages countain enough English words (like PRINT) to make them fairly easy for an English speaking human to learn.
BASIC was designed at Dartmouth College in New Hampshire, USA in 1964, and since then it has come to be by far the most widely used computer language by beginners and hobbyists. This is largely because it is very well adapted for on-line use where the user types something in and the computer answers straight away. There are other languages - such as ALGOL (in fact a whole family of ALGOLs) and PASCAL - with a much neater structure and greater power than BASIC, but only a few - such as the relatively unknown APL and POP-2 - are as easy to use on-line. Some others that must be mentioned are FORTRAN, PL1 and COBOL.
Many personal computing magazines publish programs in BASIC, and are well worth looking through for ideas. You will almost certainly have to adapt them slightly because every computer that uses the BASIC language has its own dialect, different from all the others.