If you're using a standard word processor for writing, check out Scrivener from www.literatureandlatte.com
Scrivener has all sorts of features to help writers organise their work, measure word count and set targets, keep notes, backup copies of taking snapshots to back up their work before making changes.
If I had to choose the most useful and cost-effective tool for writing a lengthy work – a novel, a thesis, a book – this is it. It comes for Mac and PC. And if you use Dropbox, you can sync it with the phone app.
You do need to devote some time up front to get to grips with the basics. There is a video tutorial that is well worth watching, and a user forum. Once you get started, you'll keep finding new features.