Collector is a free app.
You can find it on Apple, Google and Windows store.
What you need then is an organizational account for ArcGIS Online or Portal.
But you can start downloading it now and use the demo license for testing.
The Username and Password will be the same as that of your ArcGIS Organizational Account (For trial account, use the username and password of the ArcGIS Online trial that you have generated, if any).