I've read a rule some time ago that you shouldn't start a sentence from "want". But then I've seen such sentences and now got a bit confused! Could you, please, tell me which variant is correct! thanks!

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Technically, you probably shouldn't start a sentence with "want" (neither should you start a sentence with "and"), or at least, that's what we got taught in primary school at least. Although native speakers actually tend to do so, it actually makes reading harder and should be used as a continuation of a sentence rather than the start of a new one. For example "ik ga naar we winkel, want het brood is op" (I'm going to the store, because we finished the bread) is more fleunt and correct than "ik ga naar de winkel. Want het brood is op." If the sentence is short you don't really need to put a comma before the "want" either, but I think it makes it easier to understand and more logical. In English, you wouldn't really start a sentence with "because" either. However, there is an exception where, if somebody asks you something, your reply can start with a "want". For example "waarom ga je naar de winkel?" "Want het brood is op", although "want" can be left behind here all together.