Everything in Trace.Space is an item: a requirement, a test case, a chapter heading. Items are arranged in a tree, and the tree in the left sidebar is where you build that structure.
Create the first top-level item
In the left sidebar, find the Items section and click the ... button next to it, then New item. The New item button at the bottom of the tree does the same.
Pick an item type from the list, or No type for a plain item.
Type a name and press Enter. The new item opens in the document view.
Add children and siblings
In the tree, hover an item and click the + that appears. A quick click creates another item of the same type: as a child if the item is expanded, otherwise right below it. Right-click the + to choose a different type under Add item or Add child item.
In the document view, hover the gap between two items and click the + on the divider line. Again, a click adds an item of the same type, and a right-click opens the type picker.
Repeat this for any item in the structure. To move items later, drag them in the tree; see Using the Tree View. For editing the items you created, see Creating and editing items, and to bring in an existing document instead, see Importing documents.