Can access online content from within Desktop (under File menu)
Free online training seminars:
From within the My Content area, click "Add Item" to upload form your computer, with an upper limit of 1000 features:
ArcGIS Online will walk you through the process of creating a feature service to use this data in maps.
You can also drag and drop a CSV file into a Map window.
You can also upload other file formats just to share, but they won't be mappable in ArcGIS Online: Complete list of formats
Publishing data as a service is necessary to incorporate the data into maps.