Before we cover the basics of ArcGIS Online, let's discuss your access to ArcGIS Online. Some readers will have employers with organizational accounts. From Esri:
Note
If you are not a member of an ArcGIS Online organization, you can create a public account to access ArcGIS Online. A public account is available for individuals to create and share content with limits on usage. If you already have an Esri Account, it is automatically a public account and you can use it to sign in.
That is one potential path. Another is to take advantage of Esri's free developer ArcGIS Online account. This is a limited usage account, which comes with 50 credits per month. It is an excellent way to take your first steps with ArcGIS Online.
Note
Find out more about the ArcGIS Online developer account at: https://developers.arcgis.com/en/
There are three key ArcGIS Online concepts which are important to cover: Named Users, web maps, and OAuth authentication.