switching or changing themes?

Hi there,

I'm new to wordpress, I've been reading over the documentation... there's lots. I have some html experience but never used this type of software before.

I am basically taking over a website for someone else who has left and am looking at changing the style and look of what is currently there.

Is there a way that I can work on making changes to the current website and view my changes without making it active until I have completed everything? I don't want to try something and find out it is active online to everyone.

Thank you