Nice site for engineering fundementals of electronic circuits
http://www.electronics-tutorials.ws/resistor/res_6.html (circuits, potential difference, ohms law, voltage devider etc etc.)
http://www.electronics-tutorials.ws/dccircuits/dcp_7.html (Thevenins Theorem.)
You can also check these electronic circuit fundamentals.
Lessons in Electric Circuits - https://www.allaboutcircuits.com/textbook/
Basic Concepts Of Electricity - https://www.allaboutcircuits.com/textbook/direct-current/chpt-1/electric-circuits/
Video about Basic Relationships, Concepts and Laws of Electric Circuits - https://www.allaboutcircuits.com/video-tutorials/essential-concepts-of-electric-circuits/