At the bottom of each page (if permissions have not been restricted), you will see your picture (or an icon) and an empty field with the words, "Write a comment..." As soon as you click in the box, another larger edit screen will open providing multiple options for formatting, inserting links/images, and more. To provide more horizontal room to enter comments (or replies), you may find it helpful to collapse the left menu by clicking on the middle icon located at the top right of the screen immediately below your name.
Type in your comment.
Formatting commands are available on the menu bar (e.g., Bold, Italics, Underscore, Bullets, Indent, Text Justification). In the leftmost Paragraph drop-down menu, you can also select various heading styles (e.g., Heading1, Heading3) and Quote (indents & italicizes text that you copy/paste from another source).
The Linkicon allows you to embed hyperlinks to other web content both from inside the Wiki (Spaces, Pages) and elsewhere (Web Link). You can also create your own "Link Text" to make the references easier for readers.
The Tablemenu allows you to insert a two-dimensional table of varying dimensions (rows, columns). If you do not want a header row, hold the SHIFT Key while selecting the table.
The Insertmenu allows you to upload images/attachments (see also Attachments-How to Upload & Reference), insert symbols/emoticons, and use horizontal separators. Advanced users can also use this menu to insert macros offering a rich array of special functions.