Hi Helen,
Welcome to the OutSystems community.
As you are new to OutSystems the best way to learn it is by following one of the guided paths on www.outsystems.com/learn
All training are online, free and with quizzes to test if you grasped the learning material. It is the best way to learn how to create an OutSystems application.
To answer your question, start with an app based on a new app if you want to extend or customize the features of that app.
In any other case start from scratch.
As you are new to Outsystems, if you are facing issue then please try to search the issue in forum first and then add question if you did not find it, as this question is very repetitive in last few days,
Regards,
Komal