Math & Multimedia has a terrific collection of tutorials and how-to’s on their website.
The Geogebra YouTube channel has many great tutorials as well.
Audrey McLaren is a goddess of Geogebra who is a go to person for any help you need.
Jen Silverman is a Geogebra guru and it shows.
MathHombre (John Golden) has many terrific Geogebra applets constructed and shared.
A slightly generalized ellipse inspired by @JamesTanton