@jonjojojon oh this is pretty open ended.. totally depends where it's gonna get printed, what you're gonna solder on to it, how and the order of what your gonna solder and connect. My recommendation would be start by making yourself a smorgasbord of components that you can test with a multimeter easily, like super basic, but comprehensive. Make paths for your power and ground, look up each individual components requirements like, 1 capacitor, and plan the paths to the gate, the posi and ground, then move on to a resistor, then a diode, then a transistor, a relay, a transformer, an IC. Don't try to make them work together yet, just work individually. Print a few boards from wherever you're gonna order them from, mistakes happen. Next step is do a new board but start combing components to observe their interactions. Once you figure all that out and understand how each component does it's job. You can start designing more complex boards that have paths on the front and back so they can cross each other. So basically most PCB boards have an insulation layer (back), a conductive path layer (backside circuit paths), insulation layer (middle, has holes to connect front and/or back circuit paths using solder and pins), a conductive path layer (frontside circuit paths), insulation layer (front). Most basic circuits don't even need both sides.

Most PCB printing companies have predefined board sizes and shapes. Just think of the conductive layers as wires that connect the components. So when you put holes in the insulation layers, remember to make them slightly larger than the conductive layer holes in your design so when you solder, the pins have good connection to the conductive layers.

Also, larger PCB printing companies will have a few templates they can send you for #kicad that match their printing capabilities.

@jonjojojon looks like you're a university student right now---make sure to pick up as many physics courses on electromagnetism as you can stand, because for anything beyond the simplest circuits, board design always all boils down to E&M optimization. Having a good intuition for the physics behind it is a supremely valuable skill.

For fabbing any prototype designs you make, I've heard excellent things about @oshpark . Next time I'm ready to order some boards, I'm definitely using them.