Is there certain words you can't put in the first sentence of an essay?

No. It used to be true that you should never start a sentence with a conjunction (like and, but, or, etc.) but even that is falling into disuse.

Your first sentence is fine. "But not many know” would be a BAD way to stat an essay, even if in some circles it could be acceptable as the first word in a sentence.

It should also be know and not knew, unless you’re going to say later that people now know it's not good for pimples.

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If you are wary of using these words I have a couple of suggestions for you:

Many people know that honey can be good for you, however not many know it also accelerates the healing of pimples.

Many people know that honey can be good for you. Do you realise that it also accelerates the healing of pimples?

Honey has been hailed as the master healer throughout the ages. One of its properties is the ability to heal pimples.

you cannot put slangs in the first line of your essay , you should write sensibly and write grammatically right sentences that do not start from because and buts. you can take tips from best essay writing service available on the internet

ya because its not at the beginning of the sentence. But this is wrong: 1) She is so trendy. But she doesn't rock those ugly jeans. Correct sentence:2) She is so trendy, but she doesn't rock those ugly jeans.

"Is there certain words" is a good example of words you shouldn't put anywhere.