send a string to a other app

You can use intents, as long as the other app knows how to handle the data you send it.
You can also use a general intent (like the 'Share' functionality in lots of apps), that lets the user select which app the data goes to.

Has someone a little example for me, please?

As written above this is only possible if the other app has specifically supports such a feature. In that case they should also document how it can be done.

For general sharing:
http://www.b4x.com/search?query=Share