Once you are logged into OneDrive you will be able to use the online applications of M365 to work on existing/new documents.
Create New Document
Select the Create or upload (Blue +) button on the left hand side of your dashboard.
Select what type of document you would like to create from the drop down.
Upload and Edit Existing Document
Select the Create or upload (Blue +) button on the left hand side of your dashboard.
Select 'Files Upload' or 'Folders Upload' to
Files Upload is better if you only want to move a single file at a time.
Folders Upload is best at moving all files within a folder all at once.