A good friend of mine was actually taught in school that a sentence should never begin with a conjunction. In other words, a sentence shouldn’t be started with “And”, “Or”, “But”, etc. And she’s not the only one. Many people have been taught this, even though it is complete nonsense. Even today, you will find idiots on the internet claiming that this is the proper way.
The reality is that the use of conjunctions to begin a sentence has been part of the English language for centuries. In general, about ten percent of sentences start that way. Never starting with a conjunction can also lead to confusing run-on sentences that may annoy or confuse readers. Sometimes it’s simply better to break a compound sentence into two separate ones, even if the second one starts with a conjunction.
So do begin sentences with conjunctions when there is a need, but not all the time.