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I'm starting with launch files and I couldn't find any tutorial about it.

Have you tried the roslaunch wiki page? The Tutorials section should help you get started.

In addition, I want to put an "if" inside the launch file in order to create a node only IF it doesn't exits yet. ¿How can I do it?

Roslaunch cannot check for the (non)existence of a specific node. There is partial support for conditionals (if and unless), but it is used for conditional inclusion / evaluation.