I am ensecure about how to start.
Which platform and tools are needed and wich should i use as a beginner?
Do i need to build up an iot system or are there easier ways? I want to build an app which should run and save datas even if the device is off where the app is installed on.
I was looking for ibm bluemix, but maybe i should start with android studio and can upload my code later to bluemix?
The easiest way to start for a beginner who wants to also use bluemix, is to deploy the Watson IOT Boilerplate and use NodeRed. Or there are loads of recipes you would follow, google will show you. For example: https://developer.ibm.com/tv/connect-device-watson-iot-platform/. There is also a coursera course, but it doesn't cover creating a mobile app, there are other courses do that though.