Expense application,how to start?

The Simple List used to be called a List Viewer. So you can search the forums or Thunkable documentation or YouTube for how to use that.

Data Viewer List layouts can be customized but if you are new to Thunkable I would not recommend doing that for this project.

Yes, store your data in a spreadsheet. Your project may not be fully functional because of a limitation of how Thunkable accesses data in spreadsheets (see Google Sheets data source doesn't update every time - #6 by matt_conroy). But at least you can make a prototype of an app to demonstrate how it could work.