Understanding circuits requires Ohm's Law E=I*R or Voltage = Current times Resistance. This is the first important concept and it is profound. Here's an example of how it would show up in a hobby environment:
Say you have a red LED and you find out somewhere, like the datasheet, that it will drop 1.65V (V
f spec) with a current of 20 mA (I
f spec). You have a 5V supply and you know you want to drive the LED in accordance with the numbers above. How to size the dropping resistor?
Well, the resistor has to drop what the LED doesn't (series circuit, same current through every component) so (5 - 1.65) or 3.35V at 20 mA. R = E / I = 3.35 / 0.020 = 167.5 Ohms. Well, the closest I have is the common 220 Ohms, good enough! I chose a bigger resistor to guarantee lower current. The LED won't be maximally bright but it will be bright enough.
https://www.alliedelec.com/m/d/6355b8aba0b01578df0bb7b871ceefd7.pdfThe point is, you can't even light an LED without Ohm's Law. Or, you could just remember 220 Ohms for a red LED on 5 volts.
Actually, you might try 470 Ohms and find it bright enough. Maybe even 1000 Ohms. But at least you know the lower bound - 167.5 Ohms
Right after that we get to circuit analysis and things get more complex. I really like the Real Analog program at Digilent (linked above) You might try the Electrical Engineering program at Khan Academy.