My first piece of advice would be DON'T go out and spend a fortune on expensive tools and paper until you are sure this is a craft you want to take up.
I started out with a workshop and then it was trial and error!
The basics to get started are tracing paper, a white pencil (raid your child's craft box!), an embossing mat and a medium embossing tool.
Draw a small square on the tracing paper with the pencil.
Turn the tracing paper over and work on the back.
(Rub with a tumble dryer sheet/tealight candle; this will make it easier to do the embossing).
Put your embossing mat under the paper.
Using your embossing tool, GENTLY emboss in strokes from the top to the bottom of the square.
The tracing paper will start to turn white. The more strokes you make, the whiter the paper will become.
CONGRATULATIONS! You have made your first piece of embossing.
Keep drawing different rectangular shapes and practicing.
To emboss a circle use the same principle (top to bottom of the circle) before embossing around the circle. This will stop a dark shape appearing in the middle of your circle.
Keep trying and experiment.
Good luck and enjoy.