Although you may have heard otherwise, it's fine to start a sentence with "however." You just need to know when to use a comma because "however" can mean two different things, and if you don't get the comma right, you risk confusing your readers.
If you use "however" at the beginning of a sentence without a comma, "however" means "in whatever manner" or "to whatever extent."
However hard Anne Hathaway tried, she couldn't liven up a robotic James Franco during the Oscars.
If you use "however" at the beginning of a sentence with a comma, "however" means "nevertheless"
However, I loved all seven of Anne's Oscar dresses (plus the tuxedo).
For more on how to use the word "however," including how to use it in the middle of a sentence and with semicolons, see my article here.
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