Once you have logged in, you can either edit an existing document or create a new one. You can edit the files you have from Office such as Word, Excel, and PowerPoint with corresponding functionality that you are familiar with from the programs.
You can choose between plain text file, Word document, spreadsheet, presentation, or diagram. For plain text file, the default choice is to create a markdown file (.md), but you can change this to .txt or similar if you prefer.
Once you have selected the file/document type, a new, empty document will open and you can begin working on it.
If you also share the document (see separate guide), you can collaborate on it in the same way as in Google Docs.