I'm a beginner in using node-red and i need your help.
I want to use Xcomfort APi https://github.com/oanylund/xcomfort-shc-api
in Node-red dashboard and i don't know where to start and what to use.
Welcome to the forum @MMB.
If you are an absolute beginner I would start by watching the node red Essentials videos that are linked from the node red docs. That will save you time in the long run.
Then I would start by experimenting with node red and the dashboard to understand how to do basic things with buttons and other widgets on the dashboard.
Then read the node red docs page Writing Functions and in particular the section Loading Additional Modules which tells you how to link to a nodejs module, so you can use the xcomfort module.