To add a new entry, select Library → New entry..., press CTRL + N or click on the dedicated icon of the toolbar.
Then, from the dialog window, click on the type of the entry you want.
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.