Getting to the development portal: In the top-menu of this forum, click on “Apps”
You will then see a button “Create App” on the top right.
Then you are asked to choose a starting point. Choose the big area on the right half of the screen just above “App from a spreadsheet“
Make sure your spreadsheethas useful data and is formatted correctly to be used as basis for an app: In the spreadsheet have one entity per tab and have row one contain the attribute names.