To add a new entry, select Library → New entry..., press CTRL + N or click on the dedicated icon of the toolbar.
A dialog window is displayed. By default, 5 common types of entries are displayed:
For other types of entries, click on Others. That expands the window and displays the other entry types available:
Finally, the entry editor opens and let you fill in the various fields.
You can directly create a new entry of a specific entry type by using a keyboard shortcut. We strongly recommend learning the shortcuts for the entry types you use most often, e.g. Ctrl + Shift + A for adding an article entry. See Options → Customize key bindings.