Digitalwork - I suppose you are primarily interested in analog circuits, particular in active circuits which contain transistors or integrated amplifier units, correct?
For my opinion, at first you should become familiar with some relevant terms like
- gain
- transfer function
- input and output impedances
- principle of negative feedback.
Each good working amplifier needs some sort of negative feedback and, hence, it is very important to know WHY we have feedback and HOW it works (with WHICH results).