There are many definitions of physics, however this one says it succinctly,
"Physics is the the branch of science concerned with the nature and properties of matter and energy." (Oxford dictionary)
The subject matter of physics includes mechanics, heat, light and other radiation, sound, electricity, magnetism, and the structure of atoms.
A map of physics could be thought of like this,
(image source: Flickr)
All high school physics courses (HSC, IB, GCSE, A-Levels) cover a selection of these topics on the map above, generally going from the left to the right.
Above all, it can be a very exciting journey no matter what path you take!