Ways to initiate a workflow:

Once a workflow has been linked to a document, one can start the workflow by selecting the workflow that you want from the list of workflows available for the document or item. If needed, one may have to fill a form with the information that is needed by the workflow. Based on how the workflow was configured, it might be possible to customize it and start it.

There are basically 3 ways, Manually, When new list item is added, and When a new list item is updated.