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.
Need proof? We need look no farther than Elvis:
"For I can't help falling in love with you."
Or how about a toast?
See because, as, since, for