The short answer? Try to avoid it. Most sentences can be reworked so that they don't start with a numeral.
Here's the longer answer: if you can't rewrite the sentence—if there is literally no other way out—then write out the number and move on unless it's a year, a proper name, or a really long number that would make your reader squirm.
Using numerals at the start of a sentence isn't preferable, but it is acceptable. Know your audience, and use your best judgment. If your brain feels like it's going to burst when trying to read a number at the start of a sentence, just imagine how your reader's pretty little head will feel.
Example
One hundred and forty-nine thousand people signed my petition for ABC to put Happy Endings back on television.
327,000 people bought Kanye West's latest album in its first week of release.
Happy Endings may have been an awesome show, but that first sentence definitely isn't. The written-out number is annoying, so you should go with a numeral. That's what's happening in the second sentence. Using the numeral 327,000 is much easier on the eyes… and the brain.