Hmm. I think it's very difficult to learn 'optical system design'. There are many different types of optical system, and you can probably spend a long time learning just one type.
To begin with you can get away with paraxial approximations. I'd suggest looking at some catalogues (Melles Griot, Thor Labs, Newport). These often have a bit of theory and also show the bits of kit that are usually available.
Once you've done that try solving some simple problems like collimating light from an optical fibre and imaging something with a specific magnification. Ensure that you can buy the optics for this, to ensure you are designing realistic systems.