Did you finish one of the learning paths? They already explain everything pretty good.
You can also just right click inside a module in your project explorer and click on “Add Microflow”
Please take a look to the academy section. There are realy helpful tutorials how to work with mendix and what are the parts of Mendix.
Could you elaborate what you are trying to do? Microflows are part of the modeler. So you create microflows inside the modeler. And the Mendix model runs in an environment. In your case you are probably using the free tier nodes. That is the place where you can run your model in the cloud.