If you are talking about a widget you found online somewhere and you want to upload it, we cannot do that at wordpress.com.
To add widgets, you go to your admin panel, presentation > widgets and drag the widget you want from the “available widgets” area up to the “sidebar” pane, position it where you want it and let go of the mouse button. When done, click “save changes.”
Do note that when you first start a blog, there is a default set of widgets that display and when you add your first widget to the sidebar, the default set will disappear. You then have to drag the widgets you want up to the sidebar area and arrange them as you wish, and again click “save changes” when you are done.