According to my understanding white balance is procedure that removes effect of colorful lengthening. It makes picture like it was lightened with ideal white color light. Please, correct me if I'm wrong.
For the most part, that's right, but it has a subjective element to it. White balance is used to make things "look" like they are the correct colour. Often this means, to make white things "look" white rather than to have a colour cast.
Sometimes, however, there are multiple sources of light in a scene with different colours, so it's hard for the brain to make sense of what should look white and what should look coloured, and so there is a bit of subjectivity to the choice of how to set colour balance. There are also some situations where a strong colour cast is expected, and removing it would look "wrong" - for example, shooting a sunset where you expect the clouds and land to be bathed in orange, or shooting underwater where you expect everything to be a bit blue.
So you can take a methodical approach to get "correct" white balance, but in some situations it can still look wrong - or you can adjust it until it looks right, even though technically you still have a colour cast.
Another thing is that to a certain point, the eye forgives a colour cast (your brain corrects for it), but if you exceed that point, it starts to look quite wrong. This leaves room for some creativity.
The most reliable methodical approach is to get a gray card (18% gray, typically, so it's not normally overexposed when things like human skin isn't), and place that gray card where the subject is, so that the light that would hit the subject hits the card. So in other words, put the card in front of or next to the subject, ensuring that it's in the same light as the subject. Then you take a custom white balance pointing at the card, marking that as neutral.
This may be possible for some types of photography but for other types this is impractical. You can go with auto white balance (you can always adjust later, and if you use RAW this will be lossless), or set a preset white balance based on the lighting type (tungsten, flourescent, daylight, cloudy, etc).