I am using PTC Creo 8.0 and I need to do a model about a real object ( an old calculator), and I don't know how to start the sketch.
If somebody could send, how the reference sketch looks like (for skeleton modelling) that would be a lot of help.
The pictures show the calculator.
With regard to skeleton modeling strategy, you will need to provide more details what you are attempting to use the skeleton for. To reverse engineer a master model of the molded housings of that calculator, I see no need for using a skeleton.
You can import your photos into a Creo model to use as a reference when sketching. Ideally you will get photos of a top and side view (orthographic projection views) and import them into the top and side planes in a model. You can then use these as a guide to create the sketches needed to capture the form factor in the Creo part.
Details on how to place an image on a plane:
To Insert an Image on a Datum Plane or a Planar Surface (ptc.com)
About images in Creo:
This is an example of how images from a drawing were used to create a Creo part with the drawing views imported as a reference.
I need the buttons to be able to move. How do I start?
Are you asking how to create a motion skeleton model? If so, here is a tutorial on how to get started.
Sketch a Motion Skeleton | Tutorial | PTC Learning Connector