Previously, you said "I've already uploaded a map" and later "I am not able to open my map in the webmap viewer."
So, have you uploaded a map document (.MXD) or service definition file (.SD) from ArcGIS Desktop? These maps can't be directly displayed. In order to embed a map or create a web application, you need a web map that is authored in ArcGIS Online, from the Map tab.
If you want to add your own data to the web map, you have a few options:
With a Public Account (free), you can add shapefiles, CSVs and GPX files to your web map (limited to about 1000 features total between all types).
You can add references to web services running on your ArcGIS Server.
With an ArcGIS Online subscription, you can publish your data as hosted web services.
Hope this helps,
Mike