We have several words showing causation: as, because, since, for. Be careful in your use of as to show a causal connection, however, because your readers might think as imparts its more usual meaning “while.”
Henry Fowler endorses the use of as clauses when placed at the beginning of sentences. We often see this use in British English:
To show causation, you should use because, since, and for. The strongest of these is because. As a rule, do not use a comma before the because clause, unless the sentence is long and complex.
Does since mean “because”? Of course it does, despite a popular myth. You simply want to make sure you don’t use since to mean “because” when since could imply its temporal meaning (“after”). Consider this sentence, written, say, in the 1980s.
Since Reagan became president, the Soviets have come to the bargaining table.
If you’re a Republican, you’ll read since to mean “because.” If you’re a Democrat, you’ll read since to mean “after.” Thus, if you mean “because” and want to avoid the possible ambiguity, then use because, not since.
May you start a sentence with a “because clause”? Of course you may. The “rule” proscribing this construction is no rule at all. It’s a myth. Emily Dickinson’s poem Death begins: “Because I could not stop for Death, He kindly stopped for me….”
Finally, a word about for. Though because shows a strong causal relationship between two ideas, the word for shows a less direct relationship. Please note that for is a coordinating conjunction and because is a subordinating conjunction. Thus, you may (and should) use for to start a sentence: “I went to the bar after the funeral. For I needed a drink.”
You’ll find for acting as a coordinating conjunction throughout the Bible. But you’ll also find it used by top writers in modern writing. You should incorporate it as part of your style.
Don’t hesitate to start a sentence with For. It’s a coordinating conjunction, and great writers have been starting sentences with conjunctions for hundreds of years. You can remember the coordinating conjunctions by referring to the acronym BOYFANS: but, or, yet, for, and, nor, so. You can begin a sentence with any of these. And you should.
You’ll find a complete discussion of coordinating conjunctions in the Parts of Speech section of Grammar.com. Click here for the start of that discussion.