The Electric Rules Check tool built into eeSchema will help you design working, robust, reliable circuits.
When you think a schematic is well along to being finished, do an ERC by clicking the "Perform electric rules check" button at the right hand end of the top toolbar….
(The icon changed sometime before version 4.0.4 to a little ladybird "bug"..)
The ERC, which is part of eeSchema, and checks your schematic, is a bit like the DRC, which is part of PCBnew. (DRC checks your PCB's tracks, footprints, etc.)
You can make projects which "work" even though the schematic fails the ERC and the PCB fails the DRC… but for every check failure there is an answer. You would be well advised to learn whatever you need to as you go along to get your work to pass the tests which are there to help you!
To get a circuit to pass the ERC tests, it is important that pins are properly defined. As long as you are using "standard" components, the ones supplied with KiCad, this may be no problem. but as you start designing your own components, as you probably need to, quite early in your KiCad "life", it will be one of those areas where you will have things to learn. Your components will "work" (on one level)… but will lead to ERC "error" messages about otherwise valid circuits.
One "Gotcha" to look out for: When you put a power port on a line of a schematic, also add a "PWR_FLAG" component to that line. This will help keep the ERC happy. (This is one of the things covered in the power ports tutorial.)