Hi Marie,
You have to use a formatter to toggle the editable property of text field. Here is a sample: JS Bin - Collaborative JavaScript Debugging
Documentation: Custom Formatter Functions - User Interface Add-On for SAP NetWeaver - SAP Library
I would suggest that start going though data binding section in demo kit, where you will get to know / learn many things: SAPUI5 SDK - Demo Kit
Regards,
Sai.