A Basic Add-on

Add the following to myaddon/__init__.py in your add-ons folder:

# import the main window object (mw) from aqt
from aqt import mw
# import the "show info" tool from utils.py
from aqt.utils import showInfo, qconnect
# import all of the Qt GUI library
from aqt.qt import *

# We're going to add a menu item below. First we want to create a function to
# be called when the menu item is activated.

def testFunction() -> None:
    # get the number of cards in the current collection, which is stored in
    # the main window
    cardCount = mw.col.cardCount()
    # show a message box
    showInfo("Card count: %d" % cardCount)

# create a new menu item, "test"
action = QAction("test", mw)
# set it to call testFunction when it's clicked
qconnect(action.triggered, testFunction)
# and add it to the tools menu

Restart Anki, and you should find a 'test' item in the tools menu. Running it will display a dialog with the card count.

If you make a mistake when entering in the plugin, Anki will show an error message on startup indicating where the problem is.