A good place to start would be the: Training page. The first video gives a quick insight in the entire mendix platform (the modeler, but also the environment for project management, deploying, stories etc.)
The next step should be the how-to section. To prevent from drowning in all the information I suggest to start with either the Training page video or the first how-to create and deploy your first app. They cover the same basics.
Then move to the related content (see the how to section). After that have a look at the documentation section and follow some how to's on specific elements such as quick starting UX development or... (what suits your interests best). Some topics are also covered in webinars. But they also cover more advanced subjects such as widget development and REST services..
This all assumes you want to lean Mendix via self-study. Mendix offers also a number of classroom courses which will get you on your way.