I've been told that it's bad to start sentences with "this" or refer to ideas as "this" because it leaves the reader to guess what the writer means by "this".
I don't think starting a sentence with "this" is bad at all. But there is a reason why you have been told that, I suppose. You always need to be mindful that "this" may not have a clear referent, and, if not, use something else.
It's often quite clear from the context what "this" refers to. At times, you may want to use "this" as a summary of several sentences without repeating them all. Or "all this," or "such considerations," "these things," etc.