Do not start a sentence with an expletive. I am not talking about bad-words here, I am talking about the word there or it followed by is or are. These are technically called expletives, or false subjects. Expletives are not grammatically incorrect. They are just boring and wasteful. Two of the most important words in any sentence are the subject and the verb. By starting with an expletive, you waste these two prime locations on content-free words. Sometimes even passive voice is preferable to starting your sentence with an expletive, if only to get the important nouns and verbs into the subject and verb positions. Sometimes the presence of an expletive at the front of the sentence can indicate that you have not included enough content in the sentence and have merely stated that something exists. Check out these upgrades to a sentence in which the expletive (ahem) has been deleted.
Instead of: There are three buttons at the bottom of the dialog box. [things existing]
Try: Three buttons are at the bottom of the dialog box. [important noun as subject]
Or: The dialog box has three buttons. [a different important noun as subject]
Or better: The dialog box offers three choices. [upgrade verb to an action, upgrade “buttons” to state what they mean to the reader]
Or even better: Click one of the three buttons at the bottom of the dialog box. [action verb using direct address to talk directly to the reader]
Or in the order a reader will need the information: At the bottom of the dialog box, click one of the three buttons.
Or giving more detailed content: At the bottom of the dialog box, click Delete, Rename, or Ignore.
It is not grammatically incorrect to start a sentence with an expletive (case in point, this sentence), but when you read over your own writing, consider whether the expletive could be upgraded to a more informative noun-verb combo, allowing you to pack more content into fewer words.